A great many of you have remarked about the problems you’ve had getting your Mac to update its firmware. For many years now, this has been limited to updates provided and installed by Apple’s macOS software installers and updaters. As there’s no other way to obtain or install firmware updates, there isn’t a great deal you can do about this.

In the past, users of many different models of Mac have encountered this, common causes being the installion of third-party storage or memory. Although for some those remain, during the last year Apple’s installers have grown steadily more tolerant. Some readers who have previously had to replace their original internal storage in order to update their firmware, have discovered that this issue appears to have been addressed almost completely.

There’s one model which still presents problems: the iMac Retina 5K 27-inch Late 2015 (iMac17,1). It seems to have updated reasonably reliably until the update intended to take its firmware from Mojave to Catalina, and matching Security Updates for Mojave and earlier. Since then, many iMac17,1 systems have remained stuck steadfastly at firmware version 170.0.0.0.0, although a few had made it to 173.0.0.0.0 before stopping.

Thanks to your help providing me with information about your iMac17,1 systems, I can now reveal that these firmware update failures aren’t the result of simple hardware differences, such as different batches of logic boards.

I received a total of 37 reports from you, containing your iMac’s current firmware and its Logic Board ID. All three IDs listed by Mr Macintosh were well-represented in each group. Here are the figures.

There were a total of 18 iMac17,1 which should have updated their firmware but didn’t. The breakdown by logic board ID is:

• Mac-65CE76090165799A – 17% (3)
• Mac-DB15BD556843C820 – 11% (2)
• Mac-B809C3757DA9BB8D – 72% (13).

Among those was at least one which is running Big Sur betas, but its firmware remains at version 170.0.0.0.0.

There were a total 19 iMac17,1 which had successfully updated their firmware to version 428.0.0.0.0. The breakdown by logic board ID is:

• Mac-65CE76090165799A – 11% (2)
• Mac-DB15BD556843C820 – 42% (8)
• Mac-B809C3757DA9BB8D – 47% (9).

Although there is a suggestion that those with a logic board ID of DB15BD556843C820 are less likely to fail to update, and those with a logic board ID of B809C3757DA9BB8D are more likely to fail, going by logic board ID alone is no reliable indicator of firmware update problems.

Over the last year or so, I’ve heard from a lot of you with this problem on your iMac17,1 systems. It can occur with any processor or graphics option, whether the RAM is still standard or has been replaced, and irrespective of internal storage. There doesn’t appear to be any consistent indicator that an iMac17,1 is prone to firmware update failure.

One possibility is that it’s a sporadic and random failure in a single component, such as the non-volatile memory used to store the firmware on the logic board. Alternatively, it could be an obscure bug in the firmware installer which simply fails to do the job properly on certain iMacs.

If this were a random hardware failure, it’s strange that it doesn’t seem to have appeared until the update to version 173.0.0.0.0, and that hardly any Macs have suffered similar failure since that update. That seems too much of a coincidence. Indeed, the pattern of failure suggests very strongly that a change was made to the firmware updater prior to the release of the 173.0.0.0.0 update which causes failure on many iMac17,1 systems, and Apple hasn’t yet addressed that bug, leaving many Macs with firmware that is now over a year out of date, and potentially problematic with Big Sur.

I invite Apple to explain.