What makes the files disappear from a hard drive (both SSD and traditional HDD)?

I had three backup copies of my computer on three separate hard drives: two traditional disk drives and one SSD. After six months without use, I decided to update those backup copies. But he found the three hard drives completely blank. Both Linux and Windows show the "unused" disks (0 bytes used) and there is no trace of the previous directory structure.

Now, I can recover the content with PhotoRec (which unfortunately will not return the structure of the folder) but this time I do not have to do it since my main copy of the data is not affected (fortunately). Instead, I want to find out how this happened in the first place. What could make the three hard drives lose data in this way?

P.S.:- I had verified the discs after the backups. The three backups were taken at various points in time using rsync. And there is no malware on my computers. The disks never connected to the computers after the backup.