'Uncorrectable errors' on internal drive?

Hello All,

When I run Techtool Pro 12 on my iMac, the 'SMART check’ indicates my internal fusion drive is ‘Reporting Uncorrectable Errors’ , and is half way between ‘pass’ and ‘fail’.

Am I correct in assuming this drive is slowing failing, and if so, will I need to replace it, or, simple reformat it and build it back up from a Time Machine backup?

Thanks in advance, for insights offered.


Bill Taylor

Replacement is your only option. It’s not clear which part of your fusion drive is failing, but there is nothing to be gained by reformatting either part as modern hard drives automatically swap out bad sectors and SSD’s will simply wear out faster with reformatting. Best you can hope for is that only one of the components needs to be replaced if you can figure out which.

Hi Al,

Many thanks for your insights, here.

From what I can tell, via TechTool Pro 12, it’s the solid state portion of the drive that is fine; but it’s the mechanical spinning part that seems to be failing, I guess.

Can they be replaced, independent of the other part?

… of course, I’m now outside Apple warranty coverage, so is this something that still needs a technician to replace, or, can a ‘keener’ do it?

I’m on a quest to find a replacement drive, and perhaps, instructions on how to replace it myself …


Could consider using an external drive.

You’ll find instructions at https://www.ifixit.com/Device/iMac_Intel where you decide for yourself whether it’s something you want to tackle on your own.

MacSales can also provide you with parts, tools and advice on replacements.

As others have pointed out, drive replacement is possible, but it is difficult and risky in an iMac. I, personally, would hire a professional to do it. If Apple won’t (or charges too much), see if there’s an independent repair shop that can do it for a reasonable price.

But a better alternative might be to simply use an external drive. If you attach a good quality SSD via USB3 or Thunderbolt, it will probably outperform your internal Fusion Drive. Clone your drive to the new SSD before it dies and start booting from it. After making sure that it all works, you can erase the fusion drive and pretend it’s not there, to prevent confusion in the future.