http://bugs.winehq.org/show_bug.cgi?id=11145
--- Comment #5 from Yolande Haneder yolande@haneder.biz 2008-01-12 08:16:57 --- It is just what I say. The directory is owned by root, even if you have the full permission.
YOU can save on a FAT because you are saving without using wine. If an installation source wants to install something there, it can only be through Wine and if you are not root the ID on wine will be the one of the user. Now wine has a check and if user ID != owner of the directory (regardless of permission), wine is throwing a fatal error and the saving fails.
Now I have series of functions (now) systematically crashing when they try to save a new temp file in a directory owned by root.
For your reference, you can call it "directory check" and as per 0.9.43 I can not even revert it anymore *arrggh*.