[SOLVED] Sharing a data partion between OS in a multi-boot system

In this computer with Windows 8, Debian 9 and Debian 10, I created a FAT32 data partition with the Gnome Disk utility, set the mount points, set the mount options to "defaults" and mount at startup.

The partition is recognize at startup by all 3 operating systems.
Windows 8 can read and write files correctly.
Both Debian 9 an 10 can read but cannot write files (unable to create file).

What am I missing?

You are probably missing permissions. FAT doesn't have the notion of file ownership. By default, when you mount a FAT filesystem on Linux, all files are owned by whoever mounted it, which is often root. You can, however, provide mount options that give files a different owner. I believe it's also possible to change files' default permissions by mount options.

As which user are you attempting to write files? What are the precise error messages? What are the filesystem's mount options on Debian?

Originally Posted by PaulExordium set the mount options to "defaults" What am I missing?

If you're the only/first user, chances are your uid and gid are 1000, try the options below in /etc/fstab:

defaults,windows_names,uid=1000,gid=1000,umask=022

berndbausch,
using GNOME Disk utility the ownership of the mounted partition is root:root by default for a FAT32 or NTFS file system, and changing the ownership is not possible:
"chown <user> /mnt/SHDATA" gives the error "chown: changing ownership of 'SHDATA': Operation not permitted"

However if the partition is formatted as ext4, the default ownership is <user>:<user>.
See below the partition layout and the fstab.

What I really would like to have is a partition that behaves like an external USB drive. Is it at all possible?

Brains,
the initial user is also the sole user. The options you suggested
"defaults,windows_names,uid=1000,gid=1000,umask=022" don't fix it, and at reboot the system is forced into recovery mode with the error message that the option "windows_names" is not recognized. Typo error?

Model: ATA WDC WD10EZEX-21M (scsi)
Disk /dev/sda: 1000GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags: 

Number  Start   End     Size    File system     Name                          Flags
 1      1049kB  630MB   629MB   ntfs            Basic data partition          hidden, diag
 2      630MB   945MB   315MB   fat32           EFI system partition          boot, esp
 3      945MB   1079MB  134MB                   Microsoft reserved partition  msftres
 4      1079MB  492GB   491GB   ntfs            Basic data partition          msftdata
 6      492GB   642GB   150GB   ext4            Debian9
 7      642GB   646GB   4000MB  linux-swap(v1)  Swap1
 8      646GB   650GB   4000MB  linux-swap(v1)  Swap2
 9      650GB   654GB   4000MB  linux-swap(v1)  Swap3
10      654GB   658GB   4000MB  linux-swap(v1)  Swap4
11      658GB   808GB   150GB   ext4            Debian10
12      808GB   818GB   10.0GB  fat32           Shared Data                   msftdata
13      818GB   983GB   165GB   ext4            Future Use
 5      983GB   1000GB  17.4GB  ntfs            Basic data partition          hidden, diag

# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/sda6 during installation
UUID=ab2227b8-08e3-4c89-a7d2-282cb4844fd1 /               ext4    errors=remount-ro 0       1
# /boot/efi was on /dev/sda2 during installation
UUID=083D-5279  /boot/efi       vfat    umask=0077      0       1
# swap was on /dev/sda7 during installation
  UUID=88a7957c-beb9-403e-8ec8-d6aa4f0f78b9 none            swap    sw              0       0
  UUID=168aa4e9-2a64-4a1b-bf1e-8ce3d3846d4c none            swap    sw              0       0
  UUID=e6a243d4-5224-497d-b4b6-77b625be13e2 none            swap    sw              0       0
  UUID=9f0408b5-3081-49d1-afc8-35863598a44b none            swap    sw              0       0
/dev/sr0        /media/cdrom0   udf,iso9660 user,noauto     0       0
#
LABEL=FUTUSE /mnt/FUTUSE auto nosuid,nodev,nofail 0 0
#
# All attempts below have failed:
  LABEL=SHDATA /mnt/SHDATA auto defaults 0 0
# LABEL=SHDATA /mnt/SHDATA auto defaults,x-gvfs-show 0 0
# LABEL=SHDATA /mnt/SHDATA auto nosuid,nodev,nofail,x-gvfs-show 0 0
# LABEL=SHDATA /mnt/SHDATA auto defaults,windows_names,uid=1000,gid=1000,umask=022 0 0

windows_names is a ntfs-3g (NTFS) mount option and does not work for FAT32. What happens when you try to manually mount the filesystem. Must be root or use sudo if enabled.

mount -o uid=1000,gid=1000,umask=022 /dev/sda12 /mnt/SHDATA

I have an ntfs partition I use for data so I don't run into the file size limit of fat32.
This is what is in my fstab and the user writes to it with out any issues from linux and windows

UUID=4A080DEC667A53DC       /data    ntfs-3g     defaults  0  0

It may be because you don't have the file sysem type in the fstab.

michaelk #8,
your "mount" suggestion does change the ownership and the files can be read and written, but at restart the ownership return automatically to root. All the various attempts at changing the mount options with the disk utility have been unsuccessful. Could it be that the utility forces the ownership to root because the size of the partition (10GB) is larger that the maximum allowed file size (4GB) for a FAT32 format?

colorpurple21859 #9,
similarily to what you suggested I've tried:

LABEL=SHDATA /mnt/SHDATA vfat defaults 0 0

but that did not solve the problem.

I ended up reformatting the partition as NTFS and using the mount options suggested by Brains #3. The easy way is to use the disk utility. Below are the screenshots for the mount options, Debian 9 and Debian 10 respectively.

FYI manually mounting does not change the fstab file nor automatically mount it at startup.