Where is the default location of the scratch file?
Root of the boot volume. [Thought I'd better scratch that - conroy]
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On a Windows system it's the temporary folder that's identified by the environment variable TEMP, assuming that's pointed to drive C: somewhere, but you haven't said which system you're running.
For what it's worth, I find it good practice to create a root level TEMP folder (i.e., C:\TEMP) with wide open permissions, and set my user TEMP and TMP environment variables to: C:\TEMP