Hi !
In order to "Free" your scratch disk, either you transfer your photos in the photo software out to another type of storage media or transfer your photos out to your harddisk; C drive.
However, preferably, is to use a separate / 2nd harddisk to store your photos
Go to Edit > Preferences > Plug-Ins & Scratch Disks
A dialog box pops and you will see the word 'Scratch Disk' above four menus.
Here's the trick. If you have only ONE Harddrive. Change all to C:\
If you have more than ONE, the first could be C:\ the next few could be your other drives.