Thank you very much. Yes, one hard drive has the documents, etc., and one has Ubuntu. The documents drive (an HDD) is mounted to /home, whereas the other drive, mounted to /, is an SSD. I cannot remember which way around that that they are, but neither drive should be unallocated - both should have ext4 filesystems, one with the operating system, the other with documents.

If /dev/sda is the documents hard drive that has gone wrong somehow, and it is set to be mounted to /home, would that possibly cause the system to fail to boot in the way described? Do you have any idea how a previously functioning hard drive might suddenly start to show as unallocated in the circumstances described?

In any event, I shall instruct my father how to run the test and see whether anything turns up.

Not being familiar with a add-on Promise controller, in non-RAID mode. All I can guess is somehow the power cut scrambled that controller which may have scrambled the hard drive also. If sda held all the documents and was symlinked to /home on sdb. Your dad or you have your work cut out for you trying to retrieve said documents from a unallocated drive. The parted magic cd comes with test disk tools for situations like this. Google will be your friend on how to recover data using testdisk on sda if I read you right.

I run a APC
Backup on my motorcycle shop and home Desktop PCs to avoid the kind of situation you and your Dad are finding your selves in right now.

Happily, there is a backup to a remote hard drive, which was put in place using Deja Dup. The system needs to be working, however, before the restore can take place, and that will be a somewhat fiddly process. I wonder whether that will be easier or more difficult than the recovery tool that you mentioned?

Thank you, incidentally, very much for your help with this.

The remote backup would be easier if you can get it working/functioning if need be.

Thank you. I think that I shall have to pay my parents a visit to fix their system in person. This is most annoying, since I had only just finished setting up their computers when the power cut struck!

Mention to your Dad about using a APC backup power supply like I mentioned. It will save both of you major grief later IMO

Ahh, thank you - will consider that.