Back in the late 70s I was a 1st-year undergrad. at a UK uni.
An RK05 drive (which held a removable disk platter) holding the final year projects crashed, the DEC engineer was called to repair it. No one was worried, there were two backups.
The first thing the engineer did was to take one of the backups, put it in the drive to see if the problem was the disk or the drive. The head in the drive had crashed so it obviously destroyed the first of the backups.
The next thing the engineer did was to properly repair the drive and put the blank disk he'd brought with him in it to verify all was well. It was.
And then as we only had the one backup remaining, he decided to copy the backup to the blank drive so we'd have two backups.
Unfortunately he didn't write-protect the source disk, fat-fingered the copy command, and copied the blank disk to the last remaining backup.
I still have the original platter which I retrieved from the rubbish bin. The head crash had gone through the oxide right down to the base metal. I keep the platter on my desk to constantly remind me of the importance of backing up correctly....