James Pearson wrote:
Could it be that the partition table has become corrupt (e.g. overwritten)?
But everything seems to be working perfectly; is that possible if the partition table is corrupt?
Yes - the partition table may have been fine when the various file systems were mounted - which could be why all looks OK now. However, if you were to reboot, you may have problems ...
Make sure you have backups before doing any changes to the partition table.
Yes, I think I backup everything important every night, with BackupPC.
What is strange is that I don't recall any episode that might have corrupted the partition table. I run smartd on this machine; and according to "sudo smartctl -a /dev/sdb" everything seems fine with this drive, no errors reported. (I'm a bit wary of running "smartctl -t" as I'm not in the same country as the machine at the moment.)