OK, I did as advised. Since I had deleted the Offline volumes on the system booted from SM2-980-C(C:), I used the dual booted SSD850-b(C:).
Indeed, the volume names of the SM2-980-C drives do differ:
\\?\Volume{81} SM2-980-C(B:)(Offline)
\\?\Volume{dc} SM2-980-C(B:)
Which explains the described behaviour. I had presumed that the volume labels would be used rather than the volume name. I assume each windows "invents" the volume name, to avoid the problem that multiple HDs might have the same label (windows does issue a warning in that case, but then allows correcting the label.)
There is another identification called "Volume ID" in the form 0xZZZZ-YYYY. That actually gets stored on the HDs, but when you clone a disk, that (depending on the software used) creates the same, hence duplicate, "identification", making it useless to distinguish volumes (I think). Windows also warns about duplictes here, too, the user could change the "Volume ID" with Sysinternals' VolumeID.exe to make it unique.
I looked at diskpart to look for other means to distinguish HDs, but I can not determine if there would be a suitable one:
detail disk
Samsung SSD 980 PRO 1TB
Disk ID: {EB}
Type : NVMe
Status : Online
Path : 0
Target : 0
LUN ID : 0
Location Path : PCIROOT(0)#PCI(0101)#PCI(0000)#NVME(P00T00L00)
...
It seems I'm out of luck to get around duplicate offline volumes.
So I now switched to automatically removing offline volumes.
Unfortunately, this can't be limited to "removable" disks, and MS does not allow changing Removable to Local. This is highly impractical, because all USB-connected drives become removable (and so are some fixed disks, for whatever reason). USB drives, would be the most likely to be unmounted or just not connected. So the consequence of removing offline volumes is dire:
no more finding files on USB drives when they are intentionally offline.
(BTW: all my USBs use the same drive letter in every multi-boot configuration)
Any ideas?
Rolf
and guess what: now the non-existant files are no longer in the list.
but why can there be offline versions of the same drive as an online one 