Its likely System Restore is accessing your external hard drive.
In order to turn off System Protection you need to go to Control Panel > System,
then click on the Advanced System Settings link on the left. You can also access it
by typing systempropertiesprotection.exe from the Run option.
In the System Properties tab click to unselect the checkbox beside the disk you wish
to disable. You will get a prompt asking you if you wish to continue, and if you apply,
System Protection will be disabled for that specific drive.
What is the easiest way of determining what process is accessing my external USB drive?
Thanks.
Ross