If Acronis cannot read the original backup it’s because there is an error on the disk, like a bad sector or multiple bad sectors.
Please note: before running backup utilities like Acronis you should run chkdsk first.
You could try running chkdsk :x /f where x = drive letter of external drive. If there are bad sectors it will indicate them or tell you it cannot read them.
A better solution is to use a dedicated disk utility like the Western Digital diagnostic utilities. They just tell you the drive is bad. I have seen WD diags certify the drive to be good and on subsequent runs label it bad. That’s not the fault of the diags it’s the nature of bad electronics or surface faults on the drive.
To get a visual of the bad sectors, try a free hard drive utility like Victoria or MHDD. Be very careful when using either, you need to understand what you are doing and never press keys to see what they do. Either one will tell you whether surface defects are unreadable or hard to read.
chkdsk :x /r is a bit more drastic. It forces chkdsk to scan the entire surface and do automatic repairs. That can either solve the problem or ruin your disk. chkdsk :x /f is safer and it will often get a system running which is not working.
Once you get multiple bad sector errors, you’d better salvage whatever data you can real fast.
