Apple - What is the best free hard drive diagnostics app for Mac OS X?

About the only "free" tool that's worth anything is smartmontools, IMHO.

smartmontools is a free SMART monitoring application that comes from the Unix, BSD, and Linux world (OS X is based on BSD). This tool can provide extensive SMART monitoring but it's command line which a lot of people don't like. Everything else costs money - I suppose some people have actual costs, like food, water, heat, mortgage, etc. etc.

ZVH, over on MacRumors, has written a list of drive testing tools but beware most of these cost money:

Mac hard drive test software - creating the definitive list

• Disk Utility, diskutil (FREE, comes with OS)
• Smartmon Tools (FREE, Open Source)
• SMARTReporter ($4.95 for commercial version, but an older version is FREE)
• Scannerz Lite ($21.95)
• Scannerz with Phoenix and FSE-Lite ($39.95)
• Disk Tools Pro ($79.99)
• Disk Warrior ($99.00)
• Drive Genius ($99.00)
• TechTool Pro ($99.99)

That's actually a really good report. Interesting that nearly no one commented on it. In any case, I think you get what you pay for. Tools like Disk Warrior and Scannerz are known to do very limited things but do an extraordinarily good job of doing it, while other "Swiss Army Knife" tools apparently do a decent job, but not really all that thorough. The author only identifies a few applications as "free."

Although I find SMART status useful, I wouldn't base my life around SMART reports and would recommend that people do web searches about the reliability of SMART testing before using it as a defacto standard.

I assume you are aware of the Shareware tools (Like the five mentioned in this article).

In any case, since you want the free option, you will have to rely in your common sense and the tools you have (this is also true for the Shareware options, which really don’t seem to add much at this stage). Most of the benefit of TechTool Pro and DiskWarrior is to have am emergency plan in place in case of hard drive failure. But that is to be done before the problem :)

Back to your case, I suggest you give Onyx a try to check the status of your HDD’s Smart status. Assuming you don’t hear strange noises in the drive, performing a full Clone to an external drive using Carbon Copy Cloner would provide you with an important piece of information: the computer can read your entire drive, block by block.

Both Onyx and OS X can verify your volume so I suggest you also perform a verify (using Disk Utility for example) and verify your permissions.

Finally, use AppleJack to execute some of the above tests to make sure that the OS is in Single User.

If after performing all of the above, you see nothing “out of the ordinary”, you don’t hear “strange click noises” or you don’t detect any strange slowness in your drive, you can assume that the drive has not suffered any damage.

But, all things said, I’d keep my backups up2date just in case. (You do have backups, don’t you?)

I’ve had drives fall from a desktop to the floor and survive for years without any problems, and some drives tipped 0.5 inches and instantly died. Hard drives are like unicorns. :)

If you haven't already, you should pop open Disk Utility, select your drive, and click the Info button. Ensure that the SMART status is "Verified".

Of course if you hear any audible clicking or anything of the sort, then you'll likely want to order a new HDD now (then again, I figure you haven't heard any clicking, and are wanting to check the health of the drive to ensure it is okay).