A folder caller
LOST.DIR keeps appearing on my external drive. What is it / how can I make it go away?
LOST.DIR is something mac disk repair looks for and will ask about as part of it's repair process. If it's related at all to the standard
lost+found, then it's a special file that holds all the corrupted or orphaned files found during a file system check.
My guess is Wil is right, something is checking your drive and finding a problem it wants to make note of in LOST.DIR. I think this sort of thing is more likely to happen with Macs on non-HFS drives.
If I am wrong, someone else please correct me - but my guess is that upon connecting your external drive, OSX is doing some sort of scan and detecting corrupt / partly deleted files and it automatically attempts a recovery.
I would advise that you do a full disk scan on it and check for bad sectors.