I Tried to Format My External Hard Drive in Mac OS & Now It Does Not Work

Hard drives' dimensions have shrunk as their capacities have risen, but their potential for trouble remains unchanged.

Reformatting a hard drive helps prepare it for reuse, wiping out old data you no longer need or an operating system installation that no longer works properly. But if you've taken the time to reformat an external drive that's connected to your Mac, only to discover that your drive no longer functions correctly, your best course of action involves step-by-step troubleshooting.

Wrong Format

The Mac OS gives you the ability to format hard drives for use on a variety of computers and operating systems, so just because you formatted your drive while it was connected to your Mac doesn't mean you wanted to use it on that specific computer. In addition to supporting the Globally Unique Identifier, or GUID, partitioning scheme used on Intel Macs, Apple's Disk Utility can also erase, format and partition hard drives for use on a PowerPC-based Mac or with a Windows PC. If you choose the wrong scheme for your intended drive usage, you may experience problems, especially if you want to install a bootable operating system on your external drive. For example, Intel Macs won't boot from a drive that's not GUID formatted, even if the drive contains a valid operating system installation.

Hardware Problems

Your drive may have behaved properly throughout the process of reformatting it, but that doesn't stop it from failing after you finish erasing it and before you begin using it again. You can use Apple's Disk Utility application to test your drive. If its hardware accommodates the Self-Monitoring Analysis and Reporting Technology, or SMART, hard-drive diagnostic protocol, its health-status report shows up in the First Aid tab of the Disk Utility interface. If you're still unsure of the drive's reliability, you can test it with a third-party utility or pay a technician to check it out for you.

Drive-Case Problems

Your hard drive itself can be in perfect working order and appear to be malfunctioning because of problems with the external case in which it's installed. For example, a case with a failing power supply can make a drive look like it's failing. If the drive is out of warranty or you purchased the case and drive separately and assembled the combination yourself, you can test the drive while it's connected directly to a data bus on your computer. Remove the drive from its case and install it in an empty drive bay inside your Mac, or have a technician do the work for you if you don't know how. If you bought your drive in the case and the combination remains under warranty, you may want to consider returning it for an exchange or credit, because opening the case may void your warranty coverage.

Cable Trouble

Bad cabling means poor data transfer. Hard drives demand a clear path through which to send and receive information, and if the cable connecting your external drive to your Mac begins to fail, the result can look like a drive failure. If you have a spare cable on hand, reconnect your drive case using a different interconnect and see if your problems disappear. Likewise, if your drive case features more than one form of data connection -- both FireWire and USB, for example -- try connecting your drive using an alternate port. For safety's sake, it's a good idea to shut down your Mac before you recable external drives.

References

• OSX Tips: Using Disk Utility: Format, Erase, or Reformat a Drive
• Apple Inc.: Using Disk Utility to Verify or Repair Disks

Resources

• Mac OS X Lion: The Missing Manual; David Pogue