Terrible performance on Mojave with late 2013 iMac - trying (and failing) to downgrade to High Sierra

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Terrible performance on Mojave with late 2013 iMac - trying (and failing) to downgrade to High Sierra

Recently I have noticed a serious degradation in performance on my late 2013 iMac (running Mojave), to the extent that it is now completely unusable. I get regular spinning wheels, but in Activity Monitor there is no sign of any undue load on CPU or memory consumption. In even the simplest operations the machine just locks up.

Initially I wondered whether there were issues with syncing services (DropBox, iCloud Drive) but disabling them has had no effect.

Next I tried a clean reinstall of Mojave (following a backup to Time Machine), again to no avail, so I wonder whether it is an issue with Mojave itself. Unfortunately I am not able to restore from the Time Machine backup (not a major issue as valuable assets are backed up elsewhere).

So now I am trying to downgrade the installation to High Sierra to see if that fixes the issue. I have managed to create the bootable installer on a USB and have booted the machine from that.

The issue I have is that I cannot install High Sierra as it says the Macintosh HD is locked. After yet more searching I found that to get round that I needed to erase the disk in Disk Utility. I tried that but it sits there for hours doing nothing, stuck at "Deleting Volume". I get the same behaviour as in this post, where it is suggested that APFS is not suited to HDD, however I do not get the option to choose Mac OS Extended (Journaled), only APFS formats.

I have read that it is possible to manually remove the APFS container using Terminal, but before I get too deep into that I wanted to ask anyone's advice on how best to proceed.

Any insights/advice would be much appreciated, before the machine gets thrown out of the window...

iMac 27", macOS 10.14

Posted on Apr 28, 2019 9:44 AM

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Answer 1

Old Toad

Level 10

210,437 points

Apr 28, 2019 10:15 AM in response to Hoppers

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

Before running Etrecheck assign Full Disk Access to Etrecheck so that it can get additional information from the Console and log files for the report:

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Answer 2

Hoppers Author

Level 1

8 points

Apr 29, 2019 1:24 AM in response to Old Toad

Thank you very much for the advice OT, that sounds like an extremely helpful tool, I will check it out.

In the end I managed to find a way to get past my blocker, booting into Recovery mode with Command-Option-R took me back to the original Disk Utility, which detected an issue with the Fusion drive. It subsequently reformatted is as Mac OS Extended (Journaled), and I was able to install High Sierra with no problems. The machine is now running fine.

Perhaps there was an issue with the Fusion Drive and APFS - regardless all is working as expected now.

Thanks again for your help, I will post again if the issue resurfaces

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Terrible performance on Mojave with late 2013 iMac - trying (and failing) to downgrade to High Sierra

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