Catalina 10.15.3 freezing and random restarts on Late 2015 iMac


macrumors 6502a
Original poster
May 19, 2012
Galatians 3:13-14
Hi all,

I'm looking for help with issues on a Late 2015 iMac (21.5") running Catalina 10.15.3. This computer belongs to a friend of mine.

I've been helping him with video editing and was endeavoring to do so again yesterday evening, but on arriving I found out that he was having computer issues that started either yesterday morning or last week (not exactly clear on the timeframe, but it was very recent).

Anyway, the iMac was randomly restarting. At the time of my visit the computer was also moving very slowly, taking several minutes to complete basic tasks like opening a window in the Finder or executing a keyboard command. He was going to send it in for repair/replacement but when I discovered that he didn't have his files backed up I strongly recommended doing a file backup first. He offered to let me take his iMac home with me to back up the files. So I did.

Once home, I setup the iMac and plugged in my USB hard drive dock with a 3.5" HDD. Having already reset SMC and NVRAM, once the computer booted up, I began the copying process and it was behaving normally. So, I left the computer alone to do its thing (several hundred gigabytes were going to take "About 2 hours" to complete). I came back after some time and the screen had gone dark so I tapped the space bar to turn the display back on, and the copying process had stopped even though the progress bar was still there. I know it stopped because the "amount of data copied so far" number wasn't increasing, and the indicator light on top of the dock wasn't flickering like it does when files are copying.

This freezing while copying files happened during a normal boot and while booted in safe mode. During one of these attempts I tried copying a smaller amount of data (about 150GB instead of about 400GB).

I ran First Aid and there weren't any issues. Then I ran Apple Hardware Test by holding down the D key on startup, and it didn't discover any issues either. (At least there didn't appear to be any hardware issues. That's good news.)

I also changed the Energy Saver settings so that the screen wouldn't go dark until 1 hour and 15 minutes (because I knew I would be back in there to check on it before that amount of time).

After resetting PRAM and NVRAM again, I rebooted in safe mode and attempted one more copy, also with a smaller amount of data. Same result.

So I'm looking into this further today in order to do two things: 1) Back up the data, and 2) Re-establish a stable iMac if possible. I say re-establish because I've worked on this computer on several occasions before and it was fine (albeit on the slow side due to a lack of RAM and a standard platter drive instead of SSD).

After researching online, it seems that these symptoms are the result of issues with Catalina. People have been reporting a variety of issues, some the same as or very similar to what this iMac has been doing.

Given my stated goals, I intend to do the following:

1) Run the iMac in Target Disk Mode to backup the files. This will bypass the OS and allow me to simply access the iMac's hard drive on my Mac for file copying. (I have a 4,1 flashed to 5,1 cMP, so running Target Disk Mode in this case requires buying a Thunderbolt to Firewire 800 adapter, as well as a Firewire 800 cable.)

2) After a successful file backup, I will try other steps such as booting into recovery mode and running First Aid again, then booting into single user mode and run fsck, and then if necessary reinstalling Catalina from recovery mode.

3) Setup a dedicated external drive for Time Machine to do regular, automatic backups.

I'm asking for help in case I'm missing anything, either in troubleshooting this issue or concerning the above steps. Is there anything I should know further or change about any of this?

If this is indeed a Catalina issue, then I wonder if 10.15.4 (and other future releases) will fix these issues...

Thanks in advance for your help. :)
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