I just bought a Seagate drive to use on a Mac I just acquired though I've always had Windows machines. I think you might be having some issue with the drive itself. Perhaps the adapter did some intereference on ground between the pc and arduino. Here is an example: Apple can’t check app for malicious software. See the Apple Support article Safely open apps on your Mac. Seagate start here mac not working Probably not what you want.
