If you’re like me who also needs to use at least a part of the hard drive from Windows PCs, you’ll need to choose MS-DOS (FAT) format. If you’re only going to use the external hard drive with Macs or you want to use it for Time Machine backups, format it in Mac OS Extended (Journaled). Much like my WD My Passport Ultra, most external hard drives out there are customized to work with Windows, not OS X. If you experience the same issue, your only recourse is to reformat it in one of two formats. Disk Utility showed that it was formatted to MS-DOS (FAT) but I’m pretty sure it would have been NTFS instead. When I first got the hard drive, I wasn’t able to copy anything over to it (but I was able to copy from it). Why You Need to Re-Format the External Hard Drive If you’re only going to be using the hard drive on a Mac, I recommend you stick with Journaled. So to get it running, what we’ll need to do is format it in Journaled format, which is OS X only format or MS-DOS (FAT), which means it will run with both OS X and Windows. These hard drives are customized to run well on Windows (as you’d expect), and they don’t run well with OS X. Turns out, it just wasn’t in the right format. Yes, it had some software specifically made for OS X on it, but even that didn’t help. When I bought the WD My Passport Ultra drive, I was surprised to see that it didn’t support OS X out of the box.