I've read about this problem a bit and followed some instructions and still have no solution. Here's a quick run down. I have a 250 GB western digital external hard drive. I used it last night. It has all of my DV footage on it (i make films). I ejected it. I tried to use it tonight and I got the message "The disk you inserted was not readable by this computer". I went to disk utility and could see it on the left side. It was greyed out and said it was not mounted. I read that diskwarrior was the way to go. So I got that. I have 4.3. When I run it it shows the drive, but the graph and rebuild options are greyed out. I tried to follow the directions to save the info and copy it somewhere else but I can't figure it out. when I looked at the drive info in diskwarrior it says "directory cannot be rebuilt, the file system is unsupported." below that it says "this is not a macintosh disk. this disk does not appear on the desktop". Under advanced it says the drive format is "FDisk Partition Scheme" and the file system is "Windows/PC". I did get the drive originally from my brother who used it on his pc. But i don't see why this would be an issue since I've been using it for over a year on my macbook. Any ideas?