Downgrading macOS from Tahoe to Sequoia: User accounts, Time Machine, firmware

MrHoffman wrote:

TL:DR: I’d skip the downgrade, and would pursue the reported audio issues with Apple support.

Pursue the audio issues with Apple support? How? Tahoe has been out since last fall, and apparently the audio distortion issue has not yet been addressed in a bug fix update. What other remedy is there that Apple support could offer?

Yes, I'm well aware of that. And I'm hardly a computer novice, or even a Mac novice - I've owned 3 MBPs and even performed an attempted downgrade once, about 6 years ago, on a previous MBP (from Big Sur to Mojave I think it was). The operation succeeded but I never was able to return the system fully to its previous state. (I replaced it with my current machine due to issues with the battery swelling - twice, meaning the replacement battery also started to self-destruct this way - and causing a fire hazard and damaging the keyboard.) I have over 20 years of experience as a UNIX and Linux sysadmin working with a university research group, but Apple's "walled garden" is a very daunting platform for undertaking an unsupported operation like this... hence all of my questions.

Suffice it to say: I do not contemplate downgrading my OS lightly. I think I said in my OP that I knew it would be a difficult and fraught operation. But I do not see any other option at this point.

Yes I'm aware (see previous post). And no I haven't - so I shall try calling them... but this seems to be VERY much a longshot possibility, given that lots of workarounds are posted in many places on the internet... and ALL of them are unacceptable in my case.

What is the best time to call them? The last time I tried to call them (several years ago) the call volume was so high I gave up after about an hour.

Are you serious? Yes of course you can DOWNLOAD the installer - as I've already done. You cannot RUN the installer from inside Tahoe. I accidentally clicked on the downloaded Sequoia installer last night while trying to move it from the Dock - if you try that you will see.

But this brings up another question: where exactly does Software Update put the downloaded Installer? It inserts it in the Dock, but this appears to be only temporary. The Installer has (since last night) disappeared from the Dock; it's not in my Downloads folder. find (1) will take a LONG time to search through every filesystem on my machine; does anyone know where it is placed?

Thank you for the helpful details.

As it stands, I've decided to hold off trying to downgrade. I had a helpful call with an AppleCare tech person this evening, and I was able to supply her with additional info: I've found that the distortion does not occur when the sound device is a pair of external speakers connected through the headphones jack. So far I had only observed this degradation when listening through a pair of headphones connected through a third-party USB hub (needed in my setup because I have two external disks that I keep connected at all times, and the only remaining one is needed for ethernet access to the network - because my local network relies on static IP addresses and my router cannot reserve an address for a specific device on Wi-Fi).

Given this information, the tech rep recommended first trying recovery mode to force a reinstall of 26.5.1. Her working hypothesis is that the USB hub is interfering with correctly identifying and configuring the headphones now due to some change in Tahoe, or (hopefully) the OS module that performs this function in Tahoe was corrupted during the first installation. I think that's worth a try first. She also confirmed that Apple will never, or almost never, recommend a downgrade to an earlier OS. I'll keep that as a last resort, especially since she promised that if the reinstall does not solve the problem, I can reach out again and they will escalate the case. Not sure if I can dare hope that this will lead to a quicker fix of the underlying issue, but I will treat the downgrade as the "nuclear" option and try all other avenues first.

Addendum: since talking with the rep, I confirmed that if the headphones are connected directly to a USB port (bypassing the hub) the problem did not occur for at least 3 minutes, and given that when going through the hub it always occurs within less than a minute, I tend to think it only happens when going through the hub, so something has changed in the way the OS sees that hub and devices connected to it. The network cable goes through that hub though, and network connectivity and performance has been unchanged from under Sequoia.

Umm... no, that is not an option (assuming you mean a drive that connects via USB). Then I would have zero USB ports available for the external hub or the network cable, because I already have two USB ports used for external disks.

The only way I could do that is use my existing external SSD which is currently a single-partition volume that's used for the user home directories, which would obviously mean wiping and repartitioning the drive. I'm not even sure how well supported it is to boot from an external drive, and someone has already expressed doubts about whether Time Machine will correctly restore user data to an external drive.

Whether safer or not is questionable, but it's not an option for me.

I have tried Apple's suggestion of reinstalling Tahoe from Recovery mode. My hardware configuration during this installation was slightly different from the original operation: this time, my headphones were not plugged in, but the USB hub was. During the original upgrade, the headphones were plugged into the USB hub.

Findings: the distortion when playing through headphones is still there, but instead of developing gradually, it is immediately apparent. This is now true whether the sound goes through the USB hub or not, whereas before there was no distortion if the headphones were connected directly to one of the Mac's USB ports.

There is still no significant distortion on external speakers (connected via the headphones port) nor when using the Mac's built-in speakers. But I can only play audio on speakers at very low volume levels because of my living situation, so I still need a solution that allows me to use the headphones.

I am considering repeating the Recovery mode reinstall with the headphones connected directly to a built-in USB port, before asking Apple to escalate the case.

A further update if anyone is interested... I had another chat with Apple tech support and escalated the case. The senior advisor's recommendation was to have the HARDWARE checked out as he suspects that the headphones are failing [IMO a very remote possibility given the timing of events]. I've set up an appointment at the only service place in the state that Apple works with - which, unfortunately, is Best Buy, now that their only local Premier Partner is OOB.

But to correct some info in my last posting: after some more testing I discovered that most of the time, the degradation is still gradual and the behaviour is much as before the reinstall, except that it now occurs when the headphones are plugged directly into the Mac - and in the same way, generally gradually. I cannot say whether that is different from before the reinstall as I only tried connecting the headphones directly once prior to that. Even now the onset of the distortion seems to take longer with the direct connection - but that's another observation based on a limited number of tries (2).

A more extensive search also uncovered this thread on this forum, which appears to describe essentially the same problem, and also clearly related to coreaudiod, based on the logged messages the OP shared in the thread. My trigger has been different and I have not tried this with Zoom calls, but our hardware setups are extremely similar and the necessary conditions to tickle this apparent bug (audio device = USB headphones, doesn't happen with any other connected speakers) are identical.

Note the description of a gradual degradation of audio after some time - I'm paraphrasing, but that's essentially what he is describing.

I shall endeavour to log the coreaudiod output in the same way that poster did.