You know, from soon after getting up, that some days are not going to be easy.

Yesterday was one of those days. When I got downstairs, my iMac – which I normally leave up and running all the time – had shut down. Its mains socket had apparently given up delivering volts to the uninterruptible power supply (UPS), which had failed to shut the Mac down until its battery was flat.

Once I had run an extension lead from another healthier power socket, the iMac and its RAID drive powered up just fine, and everything seemed back on track. I knew that it was forcing my hand over getting the house re-wired, something we had almost got started a few years ago. But that could wait: our older daughter was visiting from the Outer Hebrides, and we were all going out, with my ageing father, for Christmas lunch.

Lunch was going to be complex, because we had a total of nine people to get to the pub (The Fighting Cocks at Arreton, here on the Isle of Wight) for noon, with just two cars. Thankfully two had elected to travel there by bus, and we all gathered on time, pulled crackers, wore silly hats, and did everything that you are supposed to do at this time of year.

We eventually got home just before four thirty, and the iMac was happily snoozing. Once I had made the cat happy by feeding her a ‘gourmet’ meal, in recompense for our absence during the day, I woke the computer and started work. Everything was fine until I opened Facebook. While scrolling down through various videos and other postings, the iMac was struck by the dreaded BSOD – the blue screen of death.

As nothing else was going to get the computer back from that, I forced it to shut down by holding the Power button. Once it had gone quiet, I started it up again. The startup progress bar got a little over half way, was replaced by a grey screen, and then, to my horror, it restarted itself – and so that went on.

I tried a ‘safe’ startup (Shift ket held down), and this time it just froze with a grey screen, refusing to go anywhere beyond that initial half of starting up.

Fearing the worst, that this was a hardware failure, I ran the online version of Apple Hardware Test. To do this, you must have an Internet connection, and hold the Option (Alt) and D keys during startup until it enters the Recovery process. I like to drive this using an old wired USB keyboard and mouse, to minimise the potential for problems. I ran the standard and extended tests, the latter taking over an hour, and Apple reassured me that the hardware is fine.

I built a bootable USB memory stick, using the El Capitan installer and DiskMaker X (donationware), but again it stopped with a grey screen just over half way through starting up from that (Option or C held during startup). I tried online Recovery (Command-Option-R at startup), which downloads a cut-down version of OS X and essential tools; just over half way through the startup process, it threw me a black screen instead of grey. And stopped.

When I started the iMac up in Target disk mode (T key held during startup), and hooked it up via Thunderbolt to this MacBook Air, I was able to access essential files on the hard drive. I even browsed its System log, and there is no clue there as to what is wrong: each startup seems to run fairly normally, until it just stops, dead in its tracks.

So I now have an iMac which shows more signs of life than Monty Python’s Norwegian Blue parrot, but still seems to be an ex-iMac. Despite its hardware appearing normal on test, I cannot but think that it must be a hardware failure, and probably one that requires replacement of the logic board. As that iMac is now over four years old, and I perhaps should have retired it sooner, for the moment it needs to be replaced.

So unless anyone has any eureka insight into a solution, I will be blogging from the confines of this neat, but sadly less capacious, MacBook Air until the delivery turns up this week. I apologise in advance for the relatively limited quantity of articles which will appear in the interregnum, but I hope to maintain their quality.

Other than getting household improvements completed before they become essential, my lessons are:

• resilience is not just for big businesses: it is for all of us;
• back up, back up, and back up;
• keep a bootable USB memory stick to hand (although it has not helped here);
• keep updated written copies of all your login details and passwords for online systems and services; I discovered that those for one mail service and Amazon (of all places) were not up to date, but have been able to recover them from my iMac’s keychain;
• UPS are not always reliable (I wonder if this is a new issue in El Capitan?);
• know how to try to revive a dead modern Mac running Yosemite or El Capitan;
• when things are going wrong, assume that they will only get wronger.

I hope that this never happens to you, or anyone else.