Occasionally we are hearing from our users that their external drives, connected via USB to their Macs, may randomly disconnect while Disk Drill is scanning them for lost data. We have to confirm that in most cases this is not related to Disk Drill at all, and in fact should be fixed by resetting PRAM, as recommended by Apple. The issue may be caused by incorrect configuration of your USB controller, hardware issues or macOS USB drivers trying to prevent further disk damage.
In some cases it might help to reset bad sectors found on this storage device by Disk Drill. Though, our regular recommendation in this and similar cases is creating a byte-to-byte DMG backup of the disk in question first, and running the recovery from the disk image instead of the original storage device.