Acronis True Image: backup cannot be started, pauses, or fails with the "Plug in external drive" error

Symptoms

In Acronis True Image, you have a backup task of your computer of some files. The backup should be saved on a USB drive that you attach to the computer.

You try to start the backup task, but it does not start. Or the backup task pauses or fails. You see this error message:

Plug in external drive <drive_letter>.

Solution

The software cannot detect your external USB drive. To resolve the issue, do the following: