You probably have Macrium's image guardian enabled on the drive, this means you can only delete images from within Macrium's program & not directly from the drive.
https://knowledgebase.macrium.com/display/KNOW7...
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.