Troubleshooting an endlessly-rebooting mac

A little more info:

• Holding command-R during boot doesn't have much effect: it just causes the same reboot to occur, but this time without the "your computer restarted because of a problem" message.
• Holding D during boot, just results in a gray screen (waiting 10min with no effect)
• Holding Alt successfully takes me to a startup disk selector -- and the computer is able to connect to wifi --, but I have no other startup disks to try choosing than my main one, and selecting it results in an immediate "your computer restarted because of a problem"

Hi,

Thanks. I’ve been delayed waiting for a disk to arrive for a backup (via target disk mode). Unfortunately, this is not going well either.

Here’s the latest:

• I was able to mount my computer using target disk mode, but it was so slow that backup is not possible. (I.e., in the finder, when I click on a directory name, it takes 5-10 min to even display the contents of that directory.) So backing up is not an option right now. (I’m hoping not too much has happened since the last successful time machine backup.)
• I have still not managed to successfully create a *bootable* Mojave external disk.
• I was able to download the installer on another computer (maddeningly it won’t run on a Catalina computer, even to install the OS on another disk, but that’s a different story) and ran it, choosing a new external drive as the target. It ran.
• I then tried to boot to that disk on my broken computer, but it took me to the “install Mac OS” utility (recovery utility?) rather than a full boot. I was able to run a disk first aid and found no problems (though I don’t believe that... more on that in a sec).
• It seemed to really want to install the OS. So I tried again installing the OS to the external drive. After doing this, I was able to select the external drive as the startup disk. However....
• When I choose that external drive as the startup disk, it once again boots into the recovery utility, not the full OS.
• So, I tried — from within the recovery utility — installing the OS to the main HD (in case the problem is that some of the OS files are corrupted). This took a long time and ultimately failed due to an input/output error. (Logfile generated.)
• Happily, though, after it failed it some how have me access to a previously hidden(?) option to open a terminal window within the recovery utility (or I was just an idiot and couldn’t find how to launch a terminal previously).
• Within terminal I found that both the external and internal drives were happily mounted and I tried to start copying files. However, this failed repeatedly with an input/output error.

So I’m trying to figure out what is going on. Presumably the problem is one of the following three:

1. soft error(s) on the internal HD — like a corrupted file or incorrect configuration. But I’m beginning to think it’s not this (or at least not only this) because of the input/output errors.
2. hardware problems on the internal HD. In this case I’ll need to replace the HD. I would have assumed this due to the input output errors. However the fact that I can’t seem to actually boot off an external drive is making me second guess. (Though the OS installation process was so awful, I wouldn’t put it past the possibility that it’s somehow mis-installed on the external drive...)
3. other hardware problems with the computer. In this case it’s a write off :(

Any advice?

Thanks,

Mike

Nevermind! Apparently after a series of attempts at rebooting, I’m somehow able to boot from the external drive!

I’m increasingly believing the issue is #2 — problem with the built in hard drive —, as I have the same painfully slow behavior that never really succeeds. Next up: how to replace a hard drive in an iMac. But that’s a question for a different thread.

Thanks for your help.

I have another computer. How would I do that?

The other computer is running Catalina unfortunately. (Definitely was a bad call “upgrading” it.)

Yes, I was hoping someone in this help forum might be kind enough to point me toward directions on how to do this.