If you back your files up on the usb drive on Tuesday, remove the drive after back u the files, and get infected on Wednesday, the files on the drive obviously are not going to be infected.

If you remove the drive. Lots of backups are done on to always-connected devices.

So, if my external drive was connected to the computer during encryption, will it also be encrypted?

yes, that's exactly what he said:

> You need read-only, airgapped backups before you can consider yourself safe.