I am helping a friend with his infected computer i decided safest option was to format and make a clean windows install.
now he has files he backed up to an external hard drive, these files will most likely be infected with whatever it is that the computer had previously. i havent used an antivirus in some time since i am very careful with my browsing habits, so i am not sure anymore what is the best way to clean and recover the files that are in the infected hard drive.
Normally an infection does not attach itself to normal files ( they normally like to live in start-up, app data, or attach them selves to browsers ). I have came across a couple of variants of the cryptolocker infection that will seek out any mapped drives and populate itself on the root of those drives. Normally ( if you have that particular flavor of infection ) you will have a .html document that will open a webpage asking you to pay in bitcoin and download the tor browser.
If it was just a normal infection and not some sort of encrypting ransom-ware he probably is fine to continue using his external as normal. If you want to be careful and not risk further infection then you can use a targeted scan from software such as webroot or malwarebytes to scan the external for your piece of mind.