Its just possible that you HAVE run out of disk space or that your hard disk is extremely fragmented. Try defragmenting and try again. If you have installed the game in a non-standard directory, this might have confused it!
As far as I am aware, Darkstone limits you to 100 saves as well and this includes quicksaves. I doubt that this could be the problem, but its worth checking out.
Finally, be sure and patch up to version 1.05b (3 patches needed) to get the best out of the game
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Regards
Mouse