lost back-up and random file names

[#1]

My Daughter has an oldish desktop. It has an external hard drive hooked up to it where she stores music and pictures. WE share that across the home network and I use it as a back up using the Sync toy.
On Saturday I ran the sync toy pairs and both failed. On inspection my back up files are not there but there but have been replaced by what appear to be folders and a few files with many GB of data. they are all named in random characters ( greek symbols, boxes etc). A lot of  my daugters music and photos have gone the same way. We have salvaged what we can and have now reformatted the drive.
Avast shows a worm was detected on 26th Feb and was in the system restore for that drive. I turned off system restore before formatting. 
I've run Malwarebytes, Ad Aware and Avast on both my PC and my daughters and everyhting comes back clean.
I'm worried that the problem may spread to the other PCs on our home network. Anything more I should be doing?

[#2]

Computer viruses (and similar) these days are out for profit.  There is no profit in randomly corrupting a disk drive unless the intention is to ask for a bribe to reconstruct the data.  Could it be that your backup drive is becoming faulty and you have just obscured the problem by reformatting it?

[#3]

I should have done a check disk and tried to repair.
Just panicked to be honest. Live and learn I suppose.