Ghost virus : "ZIPPER-2778" detected in a backup copy but not in the source !

When I had performed the latest virus scan with AVAST, “ZIPPER-2778” virus has been detected in a backup file, saved on an external hard drive. This backup had been performed two months before, with Windows backup & restore utility program. During this time, my computer ran quite OK.
However, the C:\ drive virus scan don’t show any problem. Only the backup file is infected !
Intending to eradicate this virus, I’ve formatted the external hard drive, and then restart another backup of C:\ drive with Windows utility program. As before, the backup file is saved on external hard drive F:
Just later, I ran the virus scan on drives C:\ and F:\ with AVAST program.
“Zipper-2778” virus came out again, only in backup file, on F:. On the contrary, C:\ drive is free of all known viruses.

After that unsuccessful attempt, I’ve scheduled, too, a virus scan before starting up and another after. I’ve deleted the infected backup file and performed a new backup, an at last have executed virus scan. IN VAIN ! “Zipper-2778” virus came out again, only in backup file, on F:. But C:\ drive stay free of viruses.

Have you an explanation ? Is this case a false positive ?
Thanks in advance.

You don’t mention the file name and location, and presumably these are the same in the C: drive and the backup ?

If the same file name, is this a program or file that has been updated since then or not ?

It could be if there is any compression involved in the backup, this may make a slight difference.

Personally I would allow avast to move the back-up one to the chest (assuming it can) as I wouldn’t like to have any element of doubt in a back-up…

I would also suggest that you do another back-up and see if the file in C: would be detected in the back-up.

“Windows backup & restore utility program” creates a single file to bring together the full partition. The backup file has a ‘.bkf’ extension and takes up more than 95 Go.
I always schedule the backup at night, especially for avoid any difference. So, nothing could be updated meanwhile.

I will try to do another backup with “Cobian program” instead of “Windows” and will give news soon…