Scroll to bottom for summary.

A friend of mine recently brought me a late 2009 iMac 10,1 model. She had tried to update the OS to High Sierra but had accidentally corrupted her system as well as the recovery partition. Apple refused to offer any service on the machine because it was “no longer supported.”

My original plan was to use my wife’s Mac to create a bootable High Sierra usb drive to install a clean OS on the iMac. Once I got the computer booted to the MacOS installer I ran into an issue with the iMac’s hard drive.

The MacOS installer refused to install to the hard drive due to the SMART status indicating a failing drive. However, a disk integrity check showed no issues. I understand that disk integrity and SMART are checking different things, so I needed to see exactly why SMART was showing a failing drive.

Back to my wife’s Mac to install a full bootable copy of High Sierra onto an external SSD I had lying around. I was able to swap the boot drive on the iMac to the external SSD and finally get the computer to boot to a functioning OS. From there, I was able to download DriveDX and see why SMART was showing a failure. The drive had 36 bad clusters, which was enough to trip a failure flag. But was this a true failing drive, or just a minor defect in the drive?

Apparently, the iMac had been dropped while it was being transported several years ago. I suspect this may be the cause of the bad clusters. The drive had since performed without incident until the botched High Sierra update. Just to double check the capability of the drive, I performed a secure wipe of the drive, to force writes to every single sector on the disk. This was a sort of stress test on the drive since the whole process took nearly 6 hours. My guess was that if the drive was to truly fail, a 6 hour write operation would cause it to happen.

After my stress test, I was satisfied that the drive had no serious issues despite what SMART said. So, how to force High Sierra to install on it?

Googling the problem, gave me no direct answers, only advice to “never ignore SMART errors,” “the drive needs immediate replacement, so back up your data now,” and my personal favorite “You can drive a car with 4 flat tires, but you won’t get very far.” I also work on cars some too so I think the more accurate analogy would be this: You can drive a car with your tire pressure warning light on. Do you have a flat tire? Maybe. Is the car handling just fine? Yes. Does the tire look flat or feel low? No. Then the tire is just fine. The real problem is a bad sensor giving false readings. Drive on! This is of course no reason to neglect necessary precautions, like backing up your data, or keeping a spare tire and air pump with you. And always stay alert to changing conditions that might indicate an issue developing.

Back to the problem, how to install MacOS on a drive with a tripped SMART flag? The answer was found in Disk Utility. Using the external SSD to boot into the OS, I downloaded the High Sierra installer to the iMac and ran it, installing the OS to a 32gb usb memory stick. This was a very slow process taking just over an hour. I knew that running an OS from a cheap memory stick was never going to be ideal, but it didn’t need to work well, just work. After booting from the memory stick a couple times to make sure everything was working correctly, I switched back to the external SSD and booted the iMac from it.

Using the SSD as the startup disk, I went into Disk Utility and unmounted both the internal HDD and the 32gb memory stick. Then I cloned the working High Sierra system from the memory stick to the internal HDD using the Restore feature and Verified the drive. Then using the Startup Disk utility, I selected the internal HDD as the boot device. I restarted the computer and, Voila! it worked! The iMac booted and ran perfectly from it’s own hard drive. Time to take it off life support and unplug the memory stick and SSD.

After a couple reboots to install updates everything seems to be working great! Mission Accomplished

Now the short version:

To install MacOS High Sierra on hard drive with SMART errors.

Using a working Mac, format an external drive using the following settings: Mac OS Extended (Journaled) using GUID Partition Table. Be sure to select the physical device and not the volume name in the left hand column.

Download and run the High Sierra installer and select the external drive as the target and install MacOS on the external drive.

Use the external drive you just installed MacOS on to boot the broken computer. (You may want to install some 3rd party utilities to run diagnostics if needed)

From this Mac, format another external drive using the same settings as before.

Download and run the High Sierra installer and, just like before use the external drive you just formatted as the target.

After completing the MacOS setup and making sure everything is working right, switch back to the 1st external drive and boot from it.

Using Disk Utilities, erase and format the internal Hard Drive using the same settings as earlier and then Restore the internal drive using the 2nd external drive as the source. (Make sure both devices are unmounted before the restore operation or you may get an error message)

If everything goes well you can set the internal drive as the boot device and restart the computer into a full working OS.

–Notes–
This is just the order I did things in. I would image you can make the 2 bootable drives from the first working computer and save having to download High Sierra a second time. Also I think that you might be able to skip the second bootable drive entirely by doing the cloning from the MacOS Recovery mode instead of booting all the way into the OS. I just found it easier working from the full OS.