I got this error from a darcs installed on Etch:
tim@machine:/etc$ darcs record -am 'Initial checkin'
darcs failed: problem gzwriting file: _darcs/patches/20090406120803-59231-78a2a82198e9134e0cf5600d03958044b779cc47.gz-0
Google didn't turn up anything, so I used strace as a last resort. That quickly pointed out the problem, darcs couldn't read one of the files I tried to add. If you use strace, scroll up a bit and look for something like this:
open("/etc/postgresql/8.1/main/pg_ident.conf", O_RDONLY|O_NONBLOCK|O_NOCTTY) = -1 EACCES (Permission denied)
That's the file I either needed to remove from the record or add to boring (and remove it from the pending file). Or make readable.
Hope this helps someone.