Many programs look for a folder in a specific location. In Windows, it might look on the C: drive or the F: drive. No confusion. In Linux the program looks for a specific folder in a drive mounted at a particular mount point. The problem is, the first drive mounted gets the first mound point and the second drive to be mounted gets the subsequant mount point. The mount points are not consistent, thus confusion.

For example, I have my 1 TB drive mounted (but not my 250 MB drive). I create a file in LibreOffice and save it. If at some later point I mount my 250 MB drive first and my 1 TB drive second, the "recent files" menu selection in LibreOffice cannot locate the file. If I unmount both drives and then mount the 1 TB drive first, then LibreOffice will find the file.

This drive confusion is particularly problematic using Thunderbird. The profile used by the Windows version is stored on the 1TB drive. If I mount that drive first (in Linux) then all is well when I run the Linux version of Thunderbird. But if I mount the 250 MB drive first, then Thunderbird creates a new profile on that drive, downloads the emails from the server, and I have a mess.

Is there a solution to this mess? Is there a way to "force" Linux to boot a particular drive to a specific mount point?

Usually if you have drives that you mount regularly, you make entries for them is /etc/fstab specifying exactly where they are to be mounted. This is the traditional Unix way of doing mounts.

However some distros these days mount drives dynamically under temporary mountpoints, which is supposed to make like easier for the user. For example, I plugged in a pendrive about half an hour ago and Debian automatically mounted it on a subdirectory of /media/hazel. As you have discovered, this often makes life more difficult for the user, not easier.

As far as I know that does not happen if there is an entry in /etc/fstab.

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btw, what windows calls "drive" is really a partition on a hard drive.
so linux people usually call them partitions.

Use their UUID & have no more hassles.

I appreciate the help attempts, but I'm not there yet.

I knew nothing about FSTAB, UUID, etc. when I started but I have been educating myself. Here is where I stand:
DISKS identifies the 1.0 TB partition as sdc3, and the 248GB partition as sda4

cat /etc/fstab reveals that neither has an entry (as expected)

blkid reports that both partitions have exactly the same UUID (56C26CC6C26CAC45)despite being on physically different drives. They do have different PARTUUID numbers.

Both partitions are currently mounted with /dev/sda4 on media/dave/56C26CC6C26CAC452 and /dev/sdc3 on media/dave/56C26CC6C26CAC453. sda4 was mounted first. When I mount sdc3 first, the mount points are reversed.

So it seems I need to have entries in FSTAB that refer to PARTUUID instead of UUID? IF so, how? and what would I specify as a mount point?

Sorry for all the questions, but I am quite new to all this. I am, however, amazed at and appreciative of the responsiveness of the linux community!

That's not a "proper" UUID - let me guess; these are NTFS filesystems ?. If that is the case, Linux tries to simulate a UUID for you.
Best solution is probably to add a label (name) to the NTFS filesystems - you should be able to do that in Windows, else we can give you commands to do it.

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syg00 provided the final clue I needed. Thanks to everyone who chipped in - you folks are amazing!

I renamed the partitions as syg00 suggested. Then I edited my Thunderbird's "profile.ini" file to use the name I had given the correct partition instead of the generated name.

Bob's your uncle! It no longer matters which partition is loaded first. It just works.

If you ever need to add partitions to fstab in future, you can put any of the following in the first column:
* Device (e.g. /dev/sda1)
* UUID (UUID= )
* GUID (PARTUUID= )
* User-defined label (LABEL= )

By the way, important system files like fstab have their own manual pages. Use the man command to read them.

Thank you Hazel. For me, the biggest attraction of linux is the supportive community. It is very refreshing.