Win 10 mixes up hard drives and reassigns drive letters

Yes. Drive letters are virtual assignments for drive volumes, stored within each Windows registry, and follow a convention that the Windows boot drive (the one with \windows on it) is usually drive C:
Since there are no duplicates for drive letters possible within a Windows system, there are different assignments for Different Windows systems coexisting on the same hardware.
I don't necessarily follow convention. My windows 10 that I am writing this from is on drive J:

Your initial post says

Because I am not a great fan of 10 I play with it but go back to 8.1 for normal daily use

Your assignments for drive letters are different in the Windows 10 registry from those drive letters assigned in the Windows 8.1 registry.

Sorry but I'm not sure the whole picture is being presented by the OP here. When you first install Windows it'll assign drive letters to the drives it sees. If you don't like the drive letters you can always change or re-assign them through Disk Management.

What I suspect is happening is the OP "may" be switching between drives in the BIOS to switch between OS's. Something I've done frequently. If that's the case, just do as I said above... when in Windows 10, go to Disk Management and reassign the drive letters and Windows will remember them. That even, if you're dual booting, the same "fix" should apply.

Nigels said:

Because I am not a great fan of 10 I play with it but go back to 8.1 for normal daily use, the drive letters thankfully revert back to their originals...

Fafhrd said:

Your assignments for drive letters are different in the Windows 10 registry from those drive letters assigned in the Windows 8.1 registry.

And fafhrd is correct, the assignments are separate from each other, thus my suggestion.

Not sure what's causing the recycle bin error though.

Nigels said:

but I get errors telling me my recycle bin is corrupt.

Please re-read the OP's posts.

And the confusion would be self created if you didn't know what drives you had installed in your system. In this case the OP not only knows which drives they have, but how they wish them letter assigned.

The BIOS is not putting the swapped-in Windows 10 disk in the same order and position as the swapped-out Windows 8 was.

Changing the order in the BIOS may lead to further problems.

It would be possible to change the disk assignments using Disk Manager:

Firstly from the +X menu under Windows 8.1, type k to select Disk Management, and maximise the window, so that you can see all drives and details, and the layout of the disks and partitions:



Make a screenshot (+PrtScr), so you could post it here if you like, but make a copy for your reference (either print or view on another device) when you swapout to Windows 10.

Under Windows 10, open Disk manager in the same way, and compare the layout and drive assignments to those shown in Windows 8.1.

Then you need to change the assignments to be the same as in Windows 8.

Obviously, you will need first to remove the assignments for all the drives, then replace them with the required drive letters.

On all drives except C:, right-click on the Volume name in the first column of the upper pane, and select Change Drive Letters and paths... and click Remove for all the other drives that need to change assignments.

Repeat the Change Drive Letters and paths... process, this time assigning the drive letters as under Windows 8.1.

After restarting and checking that the assignments work for Windows 10 and the drives work as expected, try swapping out to Windows 8 and back to Windows 10, and all should now work the same. Keep us posted.