Udo's Techblog

When you get a new USB drive, it's usually formatted with FAT (the Windows compatibility filesystem). As long as it's not NTFS (the Windows journaled filesystem), you can usually read from and write to the drive without problems when you hook it up to a Mac. However, if you don't need Windows compatibility, it's a good idea to format the drive using the native OS X filesystem. It offers more features, better Mac compatibility and journaling to make your data safer.

Normally, you can easily re-format any drive on the Mac by opening Disk Utility, selecting the disk in question and choose erase with the "Mac OS Extended (Journaled)" option. Sometimes, this returns a "Volume Erase Failed: The underlying task reported failure on exit" message. This is a horrible unixism, because Disk Utility obviously called a command-line tool that exited with some cryptic error message so it doesn't know what to do.

First of all, there is most likely nothing wrong with your new USB disk, don't listen to the support forums. This is the thing: when your USB drive was factory-formatted, they partitioned it to be as compatible with Windows PCs as possible, so they gave it a partition setup called "Master Boot Record". And for some disk sizes, you can't put a Mac OS partition on top of that. So all you gotta do is change the partition scheme of the drive.

Anyway, here is what you can do to fix it, even though Disk Utility should know how to tell you all by itself. Go open Disk Utility, select the USB drive. Make sure not to click on the underlying partition, but on the actual disk drive entry above it. You can find it easily, because it's not only labeled with the whole disk size, but also identifies the manufacturer of the device and it's model name. After clicking on the device, select the "Partition" tab > click on the "Volume Scheme" dropdown menu > select "1 Partition" > click on the "Options" button.

Then, a dialog window appears where you can select the partition scheme. It's probably marked "Master Boot Record". Go ahead and select the "GUID Partition Table" (or "Apple Partition Map" if you want to boot off the disk with a legacy PowerPC-based Mac). Click "OK" to close the dialog and then click the "Apply" button to re-format the drive. Should work like a charm!