Upgrades and updates to OS X have seldom run entirely smoothly. Some particular releases have been notorious, and caused widespread grief, sometimes so badly that Apple has been driven to have another go, issuing a second version of the update. But just recently, with El Capitan in particular, there seems to be more of a pattern.

Some Macs upgrade and update smoothly, yet others are consistent only in their pain and grief.

My old iMac 27″ mid 2011 is a case in point. Now, thanks to Apple’s generously extended warranty, it has a new logic board, a new graphics card, a freshly initialised hard drive, a clean installation of El Capitan, and a new user. Before the surgery, every single upgrade and update to El Capitan was horrific: most required at least one forced shutdown, and several times I almost gave it up for dead. Now after the complete internal remodelling, it is just as awkward: the 10.11.3 update was every bit as bad.

The newest models, of course, seem to be most likely to be trouble-free: my new iMac 27″ 5K late 2015 was a breeze, even though it apparently updated its firmware as well.

Some older models appear to be coping fine too, so it is not just a matter of their age relative to the release of OS X.

The last time that I can recall such variation between individual systems was with the Commodore Amiga 2000, back in 1987. Even Commodore engineers admitted that this was the case, and used to select several Amigas from the production line and test them out before taking the best to use for work. I was lucky in that mine was brought over from Germany for a hardware development project in which I was involved, and was one of the chosen few which worked brilliantly; others were erratic lame dogs by comparison. I became convinced that every Amiga had its own personality.

It is very difficult for a third party to analyse or diagnose these upgrade and update problems in OS X. The most important information – whether difficulties are most common with certain models or hardware options – is only available to Apple, which naturally keeps it confidential.

Trying to analyse the content of an update delivered from the App Store is also not easy, as the update itself is removed once it has been installed. The installation records in /System/Library/Receipts and /Library/Receipts give few clues, and downloading a non-store update package from Apple’s Support site is little help either.

Even if my gut feeling, that certain models struggle much more commonly than others, is true, it is hard to explain, and harder to work around. One possibility might be to dodge the App Store download and opt for a standalone update package. Even though that may be no smoother, at least you can install it again, in the hope that it will take better the second time around. Then there are the Combo updates, which might be the answer for those updates which never seem to take properly.

I feel sure that someone in Apple knows what is going on. If only they would let us know. It must be horrific to have to update a whole lab or business full of models which are nightmare updaters.