I have already created Fedora on a flash drive and will also be adding slackware and mint shortly to flash drives. These are for my offsite situations.
I want to create those same distros on my WD Ultra.
I also want to have a partition on the Ultra for Windows Image backup (currently on a flash drive).
Now how can I put all of this together on the Ultra and use Grub2 to multiboot yet access the Windows 8.1 Image if needed (or Windows 10 image when I get my free copy)?
I've heard somewhere one needs to install the 1st Linux distro then after installing a second, third, etc. that one needs to get pieces of each successive grub and put that into the 1st grub. Is that correct and if so how is it done? References appreciated.

Presuming a standard MBR partitioned disk (not gpt), and a BIOS (not UEFI) based motherboard.

Only one distro can control the boot code in the MBR - usually the last one installed. These days most major distros (certainly Fedora and Mint) ship os-prober which will allow all other distros (and Windoze) to be automatically discovered and added to the boot menu.
Should be nothing (extra) for you to do.

Me bad! Yeh, MOBO is UEFI

Modern linux doesn't know the difference between a usb flash drive and a usb hard drive for the most part.

Just start installing what you want.

It may be possible to clone what you have but other issues may need to be resolved and the time may be best served with clean installs.

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OK on slam and bam the installs they are for the most part totally simple.
BTW windoz is on the system hard drive.
My concern is over having a backup partition on for windows on the same drive and in the first position with linuxes. I have read stories of how grub2 looses control and only windows will boot or nothing boots. Will grub ignore the stuff or lack of anything in it? I have never set up a grub install so any reference will help.
Does Grub2 on the first Linux automatically set up later Linux installs?

A re-install of windoze will replace grub in the MBR. The mere presence of windoze (or a backup/recovery) is irrelevant to anything.

When you installed to the flash drive, you (probably) built an install image (say a liveCD). This is not the same as installing a running system to a hard disk. Better explain (in detail) what you are trying to do.
Given UEFI, it is likely both Fedora and Mint will insist the external disk is gpt - and I'm not sure if the EFI boot menu will be updated to include entries for the external system(s). Never tried it.

@Jefro: I built 3 partitions (NTFS) on the Ultra 1GB for Win BU, 2GB for Fedora22 and 2GB for Mint with Windows. Started install of Fedora but the install did not recognize the partitioning just the drive.
This was not a problem with building the FD as it was the drive.
Do U know if this is corrected later in the install onto the partitions made with Winsoz? I was expecting sdb1, sdb2, sdb3, and sdb4 all NTFS and #4 unassigned.

@syg00: Win 8.1 already installed on internal HD no need to reinstall.
Don't think the FD is a Live FD. How would I distinguish Live FD from an installed one? I have downloaded apps and scripts to it if that means anything.
When I went thru the Fedora install to the FD that when it automatically built partitions (/, /home, /etc, ...) that it formatted to GPT. Is this wrong?

I'd look at the drive with gparted to graphically see what is on that drive.

Some distro's are stinkers. The installer may prevent you from damaging a current install. Not sure why it can't see this drive if you have a modern computer.

Since I installed Fedora on a flash drive I decided to install Mint-17 on one just as drill so I could see ramifications before plunking stuff on the external USB drive.
Glad I did cuz it was a total catastrophe! Was aware Mint wants to use the internal drive and glad it didn't but somehow it built its GRUB2 so that GRUB2 became persistent. Whether Mint or Fedora was p[lugged in or not.
Could not get to windows at all period all I got was GRUB2 when I booted and no list of Mint-17 either.
I tried using the Windows image built with windows to get around it but no luck (the save recovery image box was greyed out) and hence no image only the boot. Fortunately, also had my Macrium Reflect DVD which allowed recovery, finally. You really need this app.
Just glad I didn't do it to the external drive. What a FRAPPING mess. Cleaned FD with windows.
Guess I need to run SFC /scannow again, again, again.
Decided Mint-17 is not something I want on my system at all.

Thanks for the update. Good idea to test it out.