Failed to obtain the list of location permissions

Ok, so I keep getting the same error message when trying to back up to an external drive on my Windows 7 laptop.

Failed to obtain the list of location permissions

Code: 262, 156 (0x4000C)
LineInfo: 0xF35F747B3B21F7F2

Module: C:\Program Files (x86)\Acronis\BackupandRecovery\mms.exe

Code: 65,520(0xFFF0)
LineInfo: 0xBD28FDBD64EDB8BC

Message: The media is write protected