When deleting or moving backup files using Windows Explorer you may see the following error message:

This is normal behavior and indicates that the drive is protected with Macrium Image Guardian.

MIG is a new feature of Macrium Reflect v7.1 and will automatically protect local backup drives from accidental deletion and ransomware. 

If you have updated from an earlier release of Macrium Reflect, and backup drives have been protected. then you will see the following message box the first time Macrium reflect is started: