Windows Vista 32bit

c:\Sources\darcs-2.4>cabal install -w ghc-6.10.4.exe -f-curl 

c:\Sources\darcs whatsnew
No changes!

c:\Sources\darcs record
No changes!

[edit file now]

c:\Sources\darcs whatsnew
hunk ./src/Misc.hs 96
+[_^M_][_$_]
+[_^M_][_$_]

c:\Sources\darcs record
hunk ./src/Misc.hs 96
+[_^M_][_$_]
+[_^M_][_$_]
Shall I record this change? (1/1)  [ynWesfvplxdaqjk], or ? for help: y
What is the patch name? F
Do you want to add a long comment? [yn]n
Finished recording patch 'F'

c:\Sources\darcs whatsnew
darcs: mmap of '_darcs/index' failed: does not exist (No such file or
directory)

WORKAROUND:

c:\Sources\darcs-2.4>cabal install -w ghc-6.10.4.exe -f-curl
--constraint="hashed-storage<=0.4.7" --constraint="mmap==0.4.*"

This one makes darcs work for me.

I spoke to Gracjan about this on IRC.

He was using the official 2.4.1 binary from 
http://wiki.darcs.net/Binaries - and when I downloaded that and tried it 
myself on WinXP 32-bit, I got the same problem with a trivial test (make 
repo, add file, darcs add, darcs record)

Argh, this is not so easy: if you revert to that version of
hashed-storage, darcs won't work on network shares...

If I understand correctly, Petr's hashed-storage (0.4.12) should fix this.
We have a patch221 in orbit.  

Before we release darcs 2.4 we should also confirm that darcs still
works fine on network shares

Fixed in darcs 2.4.3