I have an idea that may solve, but not explain the problem.
Plug in USB drive, with D-tools off
Right-click My Computer, click “Manage”
Click “Disk Management”
Right-Click on the USB Drive, click “Change drive letter and paths”
Click “Change” Button
Suing the drop down Menu, select a letter that would not be used with D-tools is active, as in not a letter that would be used by a virtual drive…maybe H or Z, doesn’t matter.
Click “Ok”
Hopefully that letter will stick, even if you remove and reinstert the drive. If a drive letter conflict is responsible, which is kinda unlikely actually, this may solve the problem.