The strange name of the preview folder is a bug.
But Phex will still need to duplicates the file for preview to prevent access conflicts.

Concerning your full HD... AFAIK with Java 5 it is not possible to easily determine the free disk space. Therefore I think we can't really address the full disk problem until we switched to Java 6.
Java 6 is very new and I don't think we go through this any time soon, also we just switch to Java 5 recently.

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