You just use the tortoise SVN function that does it. Never mind, I need
to investigate why mkdir c:\ and mkdir x:\ behave differently. Is one a
network drive, by any chance?

No, no network drives:
for example if e: is a separate drive/partition: (Tested this on different machines, all XP):
C:>mkdir c:\
A subdirectory or file c:\ already exists.

C:>mkdir e:\
Access is denied.
No, please no large attachments to the mailing list. Your email was held
because of size, and I'm not going to allow it through.

I gathered... fair enough.