I was installing Ubuntu to the internal disk in my main machine, with all external drives unplugged for safety - then discovered I had accidentally chosen the external drive and it wasn't unplugged. (Seeing three drives not two listed as installation targets should have tipped me off, but I guess my IQ was low that day.)
The external drive, in compliance with Murphy's Law, was my backup drive with all vital current files. Poof! The last few months of work gone. Now the drive shows only the stuff I normally expect in / on any Linux machine. It mounts showing as ext4, but the disk was (I'm 98.5% sure) originally ext3.
However, the installation did not finish. Whatever files got copied, clobbered only a fraction of the disk. df reports only 1% of the space used. Maybe the bulk of my valuable files are okay, and could be recovered with some tool?
There are other questions on this site about file recovery, but many are for Microsoft Windows, or for malfunctioning disks, or some other situation. I'm on Linux with a physically healthy external disk. I'm fairly sure that the more recent and more important files are in multiple copies on that disk, so if one copy is clobbered there's hope to get the second copy.