>USED() checks to see if you (the individual user) have a given alias (not disk file) open in one of your work areas. It does not check for open files on a drive or network.
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>There are two ways to do what you want. Honestly, the prefered way is to use a third-party package like NetLib - it's the most straightforward way to deal with the problem. Another Thread member gave a solution using low level file functions, but this is not so straightforward, in my opinion, and I suspect may only indicate whether a file is opened exclusively.

FOPEN works for shared, too...whether that's good or bad depends. But it does mean that you can open exclusively if the file passes the FOPEN test.