It's sometimes a reassuring comfort to know that computers and computer-related things are entirely reliable. In that they are anything but.
As I spent about an hour looking into this problem, and now think it has been fixed, perhaps it is helpful to document the symptoms such that anyone searching might find this page and save them some time. Especially if, like me, you don't have a spare enclosure to test with and are thinking whether to buy a new one or not.
(Yes, you probably should, is the answer, if you don't want to read further.)
My enclosure was a cheap and cheerful once with USB and eSATA ports. eSATA is a bit dated these days, but is useful if you're working with old PCs that don't have USB 3 ports.
There were different failure modes depending on how the drive was connected:
When connected by USB, the drive was not recognised by Windows. That is, it did not show up in File Explorer when connected, as if it wasn't attached at all.
In Disk Management, Windows was claiming the partition was a GPT Protective Partition, whereas it was a real GPT partition, and incorrectly had a size of 16,777,216 MB, i.e. the NTFS volume limit with default cluster size, which was also incorrect.
When connected by eSATA, the drive was correctly recognised by Windows. Creating folders and, I think, copying moderate sized files apparently worked OK, though I didn't try reading the files back from the drive.
However, running the Windows 7 Backup and Restore tool reliably caused an error. Often, as soon as the backup process started to write to the drive in the enclosure, the drive acted as if it had been disconnected. That is, the backup failed, and the drive was no longer visible in File Explorer.
As far as I could tell, there were three possibilities:
- Something was wrong with Windows. Not impossible, but this felt like more of a hardware fault, especially with the strange difference in behaviour between USB and eSATA, and with observed failures when different parts of Windows were running. It wasn't just the Backup and Restore tool that was failing, but Disk Cleanup had also caused the drive to disconnect.
- The hard drive itself was failing. Not impossible either, but unlikely. Though a magnetic disk, it is enterprise-grade and hadn't had much usage. In fact, the drive's SMART readings indicated a few hundred hours of time spent powered on.
- Something was wrong with the enclosure. Probably most likely, given the enclosure was inexpensive.
A cabling issue was ruled out on the basis that both USB and eSATA weren't working, and had very different behaviour via the two connections.
Closing off the matter
In the end, not having a spare enclosure to test with, I bought a new enclosure to try out. Out of curiosity, I chose one with both USB and eSATA to test both out, in case the problem recurred.
This did resolve part of the problem: the drive showed up both via eSATA and USB in the new enclosure. On the other hand, the Windows 7 Backup tool on Windows 10 was still failing. My hunch was that the faulty enclosure had somehow got the drive into an odd state. I never figured out what exactly the problem was here. A disk check did show errors, though I can't remember whether those were able to be repaired, or whether the drive disconnected while trying to do so. Maybe there was some corruption of the file system?
In the end, formatting the drive, encrypting the drive again and then backing up finally resolved the — by-now very tedious — problem. So, not all that exciting, but if you are witnessing similar symptoms, you can probably fix it with a new enclosure.