Unable to set cache to an alternate disk drive

[fcharp]

System: Python27 and Tilestache 1.50.1, running the basic Tilestache configuration on Windows 7...

Everything works fine as long as the cache is located on the same disk as the Tilestache startup drive. In my config, this works:

"cache":
  {
  "name": "Disk",
  "path": "file://C:/temp/tilecache"
  }

But my C: drive has limited storage, so I tried this:

"cache":
  {
 "name": "Disk",
 "path": "file://F:/temp/tilecache"
  }

This fails: it goes back to the C: drive (from which I started tilestache-server).

I have every possible format for the path line without success.

What am I doing wrong?

Frederic