External hard drive is snapping/making weird sounds in sleeping mode
It's likely just the parking of heads and the disk wanting to stay in "sleep" mode. You can read more about that here.

One way to stop the disk from going to sleep (some external enclosures have extreme disk sleep settings to conserve power) is to use pmset.

For example, to prevent an attached disk from going to sleep while your device is plugged in to AC (charger) is as follows:

sudo pmset -c disksleep 0


It's of note that this prevents all disks from going to sleep, not just the external enclosure.

pmset is the program that handles all power management settings on OS X. The Power Management Preference Pane is just a front end to that utility. All settings are persistent (saved on reboot) and are actually written to a plist (/Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist).

Additionally, you could try using HDAPM (I ran this for months before I moved to an SSD).

I would suggest trying to use pmset to prevent the disk from sleeping. It works on most external enclosures, but if you find that it's not honored by your manufacturer, you may have to find an alternate method. I've found WD and some Seagate enclosures to be problematic. pmset is not a hack and should be honored by all device manufacturers; however if you find that yours is not, try heading over to their support forums. They may have a firmware update or more information.

Lastly, if you suspect the drive of premature failure, you'll have to run a proper analysis of the drive's physical health. Disk Utility is not a reliable program in this case, as it deals with filesystem structure and integrity. You will want to use either a 3rd party utility (like Disk Warrior) or better yet, the free tools supplied by the hard drive's manufacturer (which will most likely require Windows).