Acho que desatribuir a letra da unidade fará o seguinte:
Open Computer Management on the local computer by right clicking on Computer in Windows Explorer and choose Manage. Then click on Disk Management in the left hand pane.
(Or just type diskmgmt.msc into the run dialog)
Right click on the shaded area for Drive D: System reserved (or whatever the name is / letter) in the lower right hand window.
Click on Change drive letter or paths
Highlight D:(letter of the partition) and click remove
If you receive error messages that the drive might be in use or that programs might not run correctly, don't panic. It is OK to remove the drive letter. Once a letter is not assigned, the drive will not appear in Windows Explorer.
Espero que isso ajude!