External hard drives can be funny things. Sometimes, they’ll throw up errors for no apparent reason, and sometimes those reasons – though not immediately apparent – will destroy your data.
So how do you tell the difference between the occasional USB connection hiccup and the imminent failure of a hard drive?
Natively, it’s a bit difficult, but doable. Internal drives have S.M.A.R.T. monitoring software, but USB drives can rarely use that form of monitoring, so troubleshooting is much more complex. All Mac OS installations have a suite of disk tools installed by default in the Utilities folder, and you can start there.
In the Utilities folder, open Disk Utility and click the hard drive in question in the left-hand column. Note that there are disks and volumes. Disks are hardware devices that hold volumes. Volumes are sections of a disk that contain data. Even when you’re creating a disk with only one volume, it still holds a volume (the volume just takes up all useable space on that disk).
Once you click on the disk, you’ll notice that most of the right-hand section of Disk Utility is greyed out, but the key tools are available here. Click First Aid, and then click Verify Disk. This will do a check across the disk to make sure that there are no major errors, and report back if everything is ok, or something looks wrong.
If you do find some issues, you can try using Repair Disk to fix them. If the error is logical (i.e. not a physical fault on the drive itself), this option can often fix the problem and get you back to normal. As always, try to back up everything off the drive before you begin. Repair Disk is meant to be as non-invasive as possible, but it can and WILL overwrite data if it finds errors.
If the Verify comes up clean, but you’re still having problems, then there could be a physical issue going on. They come in two flavors: minor hardware hiccups and bad sectors/blocks.
Hardware hiccups can be caused by a lot of things. Check that the power supply is firmly plugged in first. I know this sounds stupid, but you’d be amazed how many times I thought an external USB drive was dying, but in reality it was just that the power cable came loose and was shutting off the drive every few minutes.
Also make sure that all cables are security seated both at the drive and on the Mac itself. I have found on more than one occasion that a USB cable worked itself lose at either the drive or hub end. Of course, checking to make sure the USB hub is working – if you use one – is also a good idea. Try plugging other devices into the hub and looking for errors or problems.
If the hardware seems ok, but you still have issues, then it’s possible that a block or sector on the drive has gone bad. This is rare these days, as most internal and external drives will automatically mark a bad sector and simply not use it anymore, but bad sectors/blocks can indeed still cause havoc if the drive doesn’t – for whatever reason – realize they’re actually bad.
Try copying everything from the drive to another drive or set of drives if you can. If there is a bad block or sector, this process will result in the drive un-mounting itself with no warning. Power the drive off and back on, and move on to the next section. If that process works, copy everything back to the drive. If both operations succeeded, but you still get other odd behavior or other problems, then the drive is most likely failing and should be replaced.
If you do have a bad block or sector and the system dropped the drive but it came back online, you may need a new drive, but there’s still a chance you can get around it. Back up everything you can off the drive. I can’t stress that enough, because we’re about to wipe the thing clean. Copy everything you possibly can off of the disk to other storage before proceeding. That means copying everything you can off ALL volumes on that hard drive, even those that seem ok. You have been warned.
Back in Disk Utility select the drive (not the volume) that you want to attempt to fix. Go to Erase, and near the bottom click the Security Options button. Move the slider to the first position away from “Fastest” on Lion, which will show text stating that the system will write zeros over all data on disk. On Snow Leopard, chose the Zero Out or 1-Pass option. Click OK.
Click the Erase button, and read the warning that comes up carefully! This will totally destroy all data on the hard drive, and consumer data-recovery tools will be unable to get it back. Make triple-sure you have the right drive and that you backed up everything you could.
As the Erase operation goes through the process, it writes zeros to each bit on the disk. This will force any bad blocks and sectors to be recognized on write, and hopefully flagged as bad so they won’t be used anymore.
If that process works without errors, then you can use the disk again. If the Erase operation fails, then the disk has indeed gone bad, and must be replaced.
Since disk errors can happen without warning, keeping regular backups is the only true way to be sure you can always get to your data. If there is a problem, you can troubleshoot, but sometimes disks die, and only a backup can get you back up and running.