Hi, Using GHC 6.8.3, the copyFile routine isn't thread safe - it crashes when two threads try and open the same file. I think that improvements were made to avoid security race conditions in GHC 6.10.1 (or the base library associated with it), and as a nice side effect they seem to have fixed the copyFile issues. However, it would be nice if someone could assure me that the copyFile issues were real but are now fixed. Replicating them is hard, but they randomly occur during a 1-hour build on my laptop, but not my desktop. I also note that copyFile starts with the process id and increments looking for a free filename. While that's fantastic if you are avoiding races with other processes (as I think the improvements were intended to address), it's not so great for avoiding races within the same process when different threads attempt to copy lots of files at once. In doing this, I suspect I've suffered lots of retries in the temporary name creation algorithm - should I be concerned about the performance aspect of this? Would an atomicModifyIORef and global temp file counter be a good idea? For now, I've got no immediate problems as I simply shell out to "cp" to do the copying - but it would be nice if the Haskell worked too. Thanks Neil