When a USB stick is present when Defraggler is started then the drive will be marked "in use". When someone, while defragementing C:, wants to remove de USB stick using windows "safely remove hardware" function it doesn't work because the drive is in use (by Defraggler). Defraggler needs to be closed to release the lock on the USB stick (i.e. first stop defragging of wait until it is finished).
It would be nice when Defraggler would not lock any drives it doesn't work on, or respond to the request to release any file handles when an attempt is made to remove the drive.
Alternatively add an unlock button or make an option to remove a drive from the view - which will need to be working while defragmenting another drive obviously.