Problem with MacOS update 10.15.7 2020-001

I just read the boot ROM is also updated by this update. (But it's possible I misunderstood.) Could be some issue there? Apparently it's nothing a normal user can fix, I guess?!?

My setup is like this: The internal (rotational) HD is a clone of the startup-external-ssd. It really seems the update screwed up those two installations. Somehow while installation the custom startup-volume is forgotten ... or it is not recognized during start up at all.

To be honest I think they really don't test setups like the ones we use. Unfortunately. But I hope I'm wrong.

;-)

...further update.

I can't seem to boot into Recovery Mode on the affected iMac either. It will startup from the internal HD (Mojave), but completely ignores requests to boot into either local Recovery Mode or Internet Recovery Mode.

Same with me - late 2013 iMac Catalina 10.15.7 - boots from an external SSD - no OS on the internal disk but did have windows on a bootcamp disk. Unless I unplug all other disks from the USB3 ports except the SSd it boots to a windows error screen. if the External SSD is the only drive plugged in it boots normally. Even with the other drives plugged in, in the starup disk settings menu it only offers the external SSD. really not sure what is going on but this is rendering my imac unusable../

Try using a old keyboard with a cable if you have one at hand. This did the trick for me, I had no chance to access any mode or perform another keyboard command during startup with the wireless keyboard neither. I blamed the keyboard for that, but maybe it's part of the problem. With the cabled keyboard I was able to get into recovery mode. Good luck!

UPDATE:

Update was installed, but external SSD still not working as a startup disk!

I was hoping completing the update would bring boot to normality - which is not the case.

The Mac keeps loading the internal drive, now having to target SSD on every startup.

Apple, please fix this!!

I am having the same issue. Late 2013 iMac with Startup Disk running on an external SSD. After updating the system boots from original internal HDD which has macOS 10.12.3 installed. System will not recognize the external SSD which is now running macOS 10.15.7.

I have two external HDDs plugged into the iMac, and the system does recognize these HDDs.

After many unsuccessful reboots, I shut it down and then unplugged the two external HDDs and left the external SSD boot drive plugged in. This allowed me to boot as normal. Then I shut it down again, plugged the external HDDs back in, and the problem is back.

Argh.

After one unsuccessful attempt at resetting NVRAM via the Terminal, I did as CapsuleFab described and disconnected all peripherals from my iMac except the SSD. I managed to get three solid chimes during a keyboard NVRAM reset and booted up with my internal HD.

I reformatted my external USB SSD to HFS+ (journaled), GUID, downloaded a fresh Catalina installer, crossed my fingers, said a prayer, and launched the installer.

Everything went as before, but this time the iMac booted up and completed the installation. I just spent 60 minutes transferring files via Migration Assistant, held my breath during the reboot to complete the setup, and I now have Catalina successfully running again on my external SSD.

I immediately received a notice that there were updates to 10.15.7 and Safari, which I summarily ignored -- I simply don't want to go through this again.

I'm afraid we have to wait, I doubt anything will happen before next year. But I hope for a surprise for christmas ...

BTW my „fix“ seems pretty stable now, that is, I connect the SSD not directly to the iMac but via a USB hub (It's from icy box). This works even without the external power supply. When I plug the drive directly in the Mac again, it is not starting up, again. This is reproducible. Maybe give it a try, if possible.

Yes, but no luck with the direkt connection after that, either. I stopped further experiments after I got it running in this configuration to avoid things get worse again accidentally.

(BTW there're definitely other changes after the update regarding sleep/ USB. An age-old error with my wacom tablet not working after wake up from sleep is now solved and it works fine.

Could be a coincidence or not. It's a little weird such a late „minor“ update changes the system this much on a basic level ... I wouldn't be surprised if something happens after a mayor update, but a security patch crashing a mature system like catalina is ... not nice.)

Well, can you explain what you mean by

...when I typed ALT during the boot sequence, a new file system appears, it is related the update software listed as one of the possible booting disks.

I'm not quite sure when you say new file system appears. Can you explain? I'll try to help.

If you don't see the white bar, and it goes to the internal drive, I think the system is kind of confused. Instead of restarting, try to install the update, shut down, and then type alt and select the external SSD manually.

Cheers, Jack

Oh, I think I understand. So basically there was a new boot option (what you are referring to as File System) when pressing Alt. I think that means the mac boot just got confused, and paused the continuation of the update by booting into the internal disk. So the update never got a chance to be applied at restart. Then, you went directly to the external disk, ending up with a non-updated macOS. The update boot option is temporarily created when applying an update - I guess Apple Developers know this stuff better obviously.

Some really complex stuff! :) Glad you were able to fix it!

Cheers, Jack

Same here.

I have been using a Samsung T5 external SSD successfully for a few months now and what a difference it has made to the speed of my late 2013 27” iMac. From hitting return on my user password it takes 3, yes 3 seconds to finish booting.

Since the 10.15.7 update a few days ago it will not boot from the external drive. I can view the external drive in finder, I can select it as a startup disk in system preferences, System Preferences tells me I have selected it but it boots from the internal hard drive. And it’s so slow!

When I hold the Option key down at startup the external drive is not listed. Only the Macintosh HD. It’s like the startup sequence waits for a while, can’t find something on the external drive, the says oh well I’ll use the hard drive to startup.