Backup files that are virus infected

There are new “Crypto” viruses out now that encrypt all files and can’t be broken without the key. The advise is to be sure you have a good back up. The file names aren’t changed. The passport software will see them as “updated” files and replace the original.
The virus says it reads all attached drives - which would include the Passport or Book drives. Any comments or solutions?

An anti-virus is recommended at all times to minimize data infections. External units are exposed, and as soon as an infected file is copied it will continue to spread across the volume depending on how the virus is coded.

The advise is always to “have a good backup”. If I use a Passbook or similar with it’s continuous backup software, I’m afraid I will get screwed if this crypto type virus hits. Should I leave it connected or just plug it in until the buffered files are copied and then unplug it? Any official advise on this?