Time machine disk icon on boot disk

Time machine disk icon on boot disk

The icon for Macintosh HD (my boot disk) shows as a Time Machine disk. There is a file .com.apple.timemachine.supported in the root of the disk. If I delete the file and restart the computer, the icon goes back to a normal HD icon. However, the .com.apple.timemachine.supported file is recreated at some point on boot because when I log in again, the file has been recreated. If then reboot again, the icon goes back to being a Time Machine one.