I have an external hard drive that I created in Lion formatted it exFat. Upgraded today to ML then plugged the drive in and turned it on, a little pop up appears saying my iMac doesn't recognize this drive. I tried repairing the permissions in Disk Utility to no avail..works fine on my laptop which is still running Lion. All my music, movies, etc is on this drive..Not sure what else to do to get it work besides maybe a clean install of Mountain Lion? I had originally just upgraded.