as the subject suggests, I am trying to multi-boot (3) distros on a single GPT disk:

• Debian 10
• Fedora 30 Workstation
• openSUSE Leap 15.1

despite being a linux newbie, I was able to get everything to install and boot properly on the first try, but I'm still dissatisfied with the end results.
there were also a few things about the process that I don't quite understand so I was hoping someone here might be so kind as to enlighten me.
I installed each distribution in the order listed above (alphabetical) and am confused by the following:

1. neither Fedora nor openSUSE liked the FAT32 EFI partition I created during the Debian installation, stating it needs to be formatted as vfat
2. both Fedora and openSUSE installers made a microSD card available to install to, even though this machine doesn't support booting from that device

I wound up with a really ugly partitioning scheme and I can't imagine I did this right:

mmcblk1p1 FAT32 EFI/esp (Debian /boot/EFI)
mmcblk1p2 EXT4 (Debian /boot)
mmcblk1p3 EXT4 (Debian root)
mmcblk1p4 vFAT (Fedora /boot/EFI)
mmcblk1p5 EXT4 (Fedora /boot)
mmcblk1p6 Linux-swap (Fedora swap)
mmcblk1p7 EXT4 (Fedora root)
mmcblk1p8 vFAT (openSUSE /boot/EFI)
mmcblk1p9 EXT4 (openSUSE /boot)
mmcblk1p10 Linux-swap (openSUSE swap)
mmcblk1p11 EXT4 (openSUSE root)

if the EFI partitions won't play nice with one another I can accept that, but I couldn't figure out how to share BIOS boot partitions or swap space between the installations.
is that not possible with the distros I've selected? what am I doing wrong?

Multiboot is as much art as science, so "wrong" may or may not describe anything you did or didn't do. Context is important.

WRT #2 I don't believe Linux OS installers are designed to determine whether a BIOS has the capability or not to boot from an MSD device, but I know both openSUSE and Fedora installers have the power to let you decide what to put where, as does Debian's, and so allow avoiding use of the MSD card entirely or in any portion. With multiboot that power needs to be taken advantage of.

Not having used Debian to create an ESP partition, I don't know what it could have done to make a FAT32 partition not VFAT. Maybe the complaint was actually about its size rather than the type. The openSUSE installer warns that 100MB is too small in an unwarranted and frightening fashion, quite unfortunate. 100MB is perfectly fine until HD size is up around 16TB and has 4k sectors exposed without 512b sector size emulation. For systems with that much storage and 4K non-emulated sectors, the minimum jumps modestly up to around 260MB.

I suggest to start over. First, determine whether and which of the three you wish to keep in control by having its Grub mention present a menu after POST. On mine that's openSUSE. Once you decide, you can make that distro the last installed and avoid a bit of extra fiddling after all three are installed.

After that decision is made, decide how you wish space to be allocated among the three, then use any partitioner you are familiar with to apply your decision in entirety. At this point you could format each of your new partitions. If so, you would then use each installer's "partitioner" merely for specifying which partition is used for what purpose. If not you would specify both what to put where as well as how to format. An example might be as follows:

1. 300MB type ESP (FAT32/VFAT)
2. swap
3. debian / (EXT4)
4. fedora / (EXT4)
5. openSUSE / (EXT4, not BTRFS, unless / is to be a copious size; due to multiboot environment, not because of BTRFS itself)

Optionally, create a single /home for all three to share as /home, which presents potential for user settings conflicts, or
as a single data partition to mount to a subdirectory under /home/, which will keep user settings specific to each distro, while facilitating easy access to data you wish easily shared among distros and/or users.

Optionally (and personally not recommended), create 3 swap partitions. This I have never done on any multiboot PC, of which I have many, among which a bunch have more than 30 distros installed. The only reason I can think of to have unique swaps is to try to hibernate every distro, which I believe is probably unworkable. If you have enough RAM, you probably need no swap, though even if not needed, it can be useful. This PC has 16GB RAM with 16+ GB allocated for swap but usually kept turned off. Debian always insists on formatting swap. To work around this foolishness, either install Debian first, or when installing Debian, specify to have no swap, then add swap to /etc/fstab after installation completes.

It is not mandatory to mount swap or filesystems by UUID. Volume labels can be assigned, by you, and used to mount by. This makes it easy for a human to sync all partitions in each distro's fstab to each other's. You pick labels you can remember and type without eyestrain or typos. These same labels can be used also to customize Grub menus.

Note that whatever size you choose for the ESP partition, only a tiny fraction of it is likely ever to be used. A distro only ever puts as few as two relatively small files there. 100MB is probably sufficient to install at least 40 unique distros.

Whether you do it before or after starting over, learn to use efibootmgr, the tool for managing who and what controls transfer of control from the UEFI BIOS to which Grub EFI. It's the tool of first choice to use to return control to your favorite distro's Grub after another usurps it following installation or update. Depending on BIOS, you may still need to change boot priorities on occasion following distro updates that update Grub. This too can be minimized, by eliminating Grub or a portion thereof from the distro you don't want to be producing the boot menu you get. If you haven't already, get familiar with your motherboard's BBS key. On my Asus it's F8, on my Gigabyte and MSI, F12. Most others I think are one of these two or F9. I minimize opportunity for usurpation to occur via modification to the fstabs of all distros except that which I want to be in control. Simply remove or comment the ESP partition line in all but the favorite's that mounts the ESP partition.

If you wish to see the same boot menu after every POST, consider doing as I do by creating custom.cfg in /boot/grub(x)/ on your favorite, to be called by a modification to /etc/grub.d/ whenever grub(x)-mkconfig is called. By copying 40_custom to 06_custom, mkconfig will put the entries in custom.cfg at the top of the boot menu. Details can be reserved for later, after you're happy about everything else about your new installations. You can preview this and alternatives in this openSUSE forum thread.

Again note, this is both science and art. There is far more than one "right" way.

Note I've never yet done a Fedora installation to GPT/UEFI. I have lots of Fedoras in multiboot, but only on MBR. On UEFI I do have multiple openSUSEs and multiple of Debian and two of its derivatives.

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thanks so much for your speedy and informative reply!

Yes, that was part of the warning message, I just assumed it had to be referring to the formatting since the EFI partition I initially created was purposely way oversized at 1 GiB and I had made the BIOS boot partition 2 GiB.

That sounds like a good course of action. This go-round was basically a trial run to become familiar with the partitioning requirements of each distro.

Honestly, my Boot Options are a mess! because of the way I did things, each bootloader is completely seperate from the other 2. meaning when I boot into Debian my only options are Debian or Debian recovery mode, etc.
I can sort of hack it and set each bootloader as a different boot option and if I want to select anything other than the first option, I can just press ESC and type 'exit' in the command-line to failover to the next boot option in order.
EXCEPT, that doesn't even work. the only way I'm able to switch between distros is to enter the UEFI menu and switch the boot option to my chose OS and leave all other options disabled. ugh.

beyond that, each OS has (2) boot menu entries to select from:

1. UEFI: Built-in EFI Shell
2. Debian
3. debian
4. Fedora
5. opensuse-secureboot
6. opensuse
7. Fedora

I'm guessing one set of entries is for 'UEFI Secure Boot w/CSM' and the other is for 'UEFI (non-CSM)' boot modes. There are no Secure Boot or CSM options in my UEFI firmware settings so I'm not sure how to interpret that. it could be that Secure Boot is forced on this system or it could be that CSM is not supported.
I don't remember which it is for sure

I do like the openSUSE bootloader the best. Debian's is graphical but looks a bit too 2000's and Fedora's is not graphical at all.

when installing Fedora, I opted to let the installer create the partitions as required. it made a 200 MiB EFI partition and a 3.68 GiB swap space, so I'll use that as a template for the next round of installs.. wish me luck!

and thanks again so much for your reply

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well, I just gave it another go.. got to the last step (openSUSE installation) and it won't accept the partitioning
I had set the EFI partition to 250 MiB when I installed Fedora and SUSE says it must be 256 or larger.. boo!

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That's the same derelict situation as when the ESP is 100M (Windows' usual size). Note that I wrote 260M, not 250M. Just dismiss the warning and proceed to use the 250M.

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I usually set my efi partitions at 300mb and have never had any issues with any distros and not run out space booting multiple distros.
When installing the distros what did you select during the partitioning stage? What I usually do is create several partitions manually before installing the first distro, then during the partitioning stage select something else/custom/? and select the partition to install the distro on. The distro I want to control grub I usually install it last on a new system.

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I got everything to work right this time. I still have to work on editing the GRUB settings. The Debian bootloader includes Fedora but openSUSE doesn't list any other operating systems. also, the Fedora installer, which I ran first (followed by Debian and finally openSUSE) wanted to put the swap partition after the root partition no matter what I did for some reason. not a big deal I guess, but it kinda irks me. partitioning is now as follows:

• 450 MiB /boot/efi
• 17681 MiB Fedora /
• 6144 MiB swap
• 17681 MiB Debian /
• 17681 MiB openSUSE /

my RAM usage in Fedora (KDE Plasma) is about 2.4/3.7GB but the swap file is at 1.4GB, does that sound right?

well, I did manually part after I booted into the LiveCD and prior to running each installer, but I only created the parts as needed. meaning when I was installing the first OS, I only created the EFI, swap and 1 root partition. when I installed the second OS, I only created one more root and just set the existing EFI and swap as their corresponding mountpoints. I thought I had read something advising against parting out an entire disk initially to minimize the risk of files erroneously spilling over into other partitions, or perhaps that's just a practice I picked up from installing Windows.

I tried resetting the partition table twice and re-parting but Fedora still put the swap partition after the root, e.g. part 3

I just reinstalled Debian because the Live CD didn't include all the drivers for the Intel 7260 wireless adapter and I wasn't able to manually install it after booting into the install. Now all 3 OS are included in the Debian bootloader, close enough for me.

The only thing I still don't get is why the EFI is formatted FAT16

Thanks for your help!

Are you sure it didn't look something like one of these?
https://en.opensuse.org/images/e/e7/150-GRUB.png
https://en.opensuse.org/images/a/a2/...lt_display.png
except to say 15.1? openSUSE may have put the others on the Advanced Options menu. That is easily fixed in YaST Bootloader, where you can select to have everything on a single menu screen, and/or add the other distros after having checked the wrong box on the subject during installation's bootloader setup. I don't remember whether it includes other distros by default, since I'm always booting installed systems from custom.cfg, and make lots of custom settings during installation.

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Fhow did fedora do that if you was using custom partitioning?

Because you didn't take control before starting. There's no reason to use any partitioner over and over and over again, or mixing things up by using multiple partitioners, instead of doing the whole job once, getting exactly what you want, unless relinquishing control is your desire. It's your PC. Have it your way.

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Again, because you didn't take control. FAT16 is legal. Some people may prefer it on so small a partition, and for various reasons. Distro installers can't be expected to figure out what everyone wants. They use formulas, so you get what they give you, unless you take charge.

Still, you seem to be learning fast, making progress more quickly than many, many other newbies.

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Yes. Firsty, there are 2 boot options for opensuse listed in the UEFI:

• opensuse
• opensuse-secureboot

If I opt to boot from either of those, the bootloader menu has 2 options:

• openSUSE Leap 15.1
• Advanced options for openSUSE Leap 15.1

If I select advanced options, I get 2 more options:

• openSUSE Leap 15.1, with Linux 4.12.14-l0151.28.13-default
• openSUSE Leap 15.1, with Linux 4.12.14-l0151.28.13-default (recovery mode)

If I use the Debian bootloader, under advanced options for openSUSE it has 6 options which all say "openSUSE Leap 15.1 (on /Dev/mmcblk1p5)"

Your openSUSE installation proceeded without the "Probe Foreign OS" box checked on https://doc.opensuse.org/documentati...er_options.png, which means grub2-mkconfig is skipping use of os-prober when building grub.cfg. You can change that via the YaST bootloader selection, or via manual edit of /etc/default/grub, commenting or eliminating or changing to false the GRUB_DISABLE_OS_PROBER="true" line.

It seems currently the only option for putting all openSUSE Grub-EFI menu selections on a single screen requires a manual edit of /etc/default/grub. Addingto it should include Debian and Fedora entries at subsequent grub.cfg rebuilds.

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