I have an internally connected hard drive. It is drive letter E. For whatever reason, Photoshop can read files on that drive normally (so can everything else). But in Preferences -> Performance the drive is not listed for a scratch disk. C, D, and G are listed and seem to be fine.
Drive E has about 600 GB free.
Photoshop CC 2014, Windows 8.1
It could be a permissions issue. You may have Read permissions but not Write permissions to that particular drive.
Or it could (less likely) be a formatting issue, as in an HD previously formatted to Mac.
That's all I can think of, as I don't do windows myself.