P: Allow use of Drive Letters A: and B:

You can import to any drive, including A: or B: by using the right-hand area of the top Import panel's "Select Destination" arrow and selecting "Other Destination". Then input the drive you want to import to, including, probably, the destination folder. You have to do the latter here because there is no way to select a destination folder using this procedure. So make or name that folder in your operating system, and select it when you are setting up the Destination drive as above.

I Imported from my camera card and from my newly named A: drive to my B: drive.

After you do this keep a lookout for your destination drive to show up on these panels. My B: drive appeared in the Source column and then disappeared. So it's still flaky, but I've been able to use Import normally with Raw, JPG and DNG imports this way. Seems neater in results, and quicker than setting up a mount point.

And if you want to use the A and B drives as the destination of the import (it wasn't clear from your post), see this easy workaround:

https://feedback.photoshop.com/conversations/lightroom-classic/lightroom-display-a-b-drive-letters-in-import-destination-on-windows-not-fixed-in-63/5f5f44ee4b561a3d4229b97f?commentId=5f5f464f4b561a3d42810408 

Windows has allowed drive letters A and B to be assigned to arbitrary disks since at least Windows XP.  LR Classic can easily import from A & B -- see my previous post. It's just a silly bug that LR Classic doesn't list A & B in the Import window's Source column.

The ill-fated redesign of the Import window in LR 6.2.1 fixed this bug, but when Adobe backed out of the fiasco in LR 6.3, the bug came back :-<

In LR Desktop (Cloud), you can easily do Add Photos for pics residing on A and B.

See this post for how to import from the A and B drives easily:

https://feedback.photoshop.com/conversations/lightroom-classic/lightroom-display-a-b-drive-letters-in-import-destination-on-windows-not-fixed-in-63/5f5f44ee4b561a3d4229b97f?commentId=5f5f47574b561a3d42ce1080 

I don't believe this is a limitation imposed by Adobe, but rather a limitation imposed by the Windows operating system. Whether or not it can be overridden with the  "assign" command is something you might consider exploring.

I have a 'Left of centre field' HDD set up.
Adobe Lightroom doesn't recognize (A:), I'll also assume (B:), for import. Could this be adjusted since we are no longer booting from 3.5" floppy disks?