|There must be a program accessing the drive. Could be your system restore settings. I don't use system restore for my external drive because it is not always active.|
There may be another reason too.
Go to Windows Disk Management and right click on the listing for that drive. Choose properties> hardware. Highlight the drive listing from the list and click properties> policies. If write caching is enabled that might be causing the message. I suggest you only check the better performance bullet.