What Was Happening
A customer in Putney SW15 brought in a 27-inch iMac that had become, in their own words, really slow. Not broken and not refusing to start — just painful to live with. Applications taking an age to open, the spinning beachball at the slightest provocation, and the general sense of a machine thinking about something else.
What made it different from an ordinary slow-Mac job was the sentence that came next. The SSD had been replaced six months earlier, and it had been replaced by us.
That changes the shape of the job. A slow iMac still booting from its original hard drive is one of the easiest calls in the trade: the storage is the bottleneck, an SSD lifts it, the machine feels new again. This one had already had that fix. So either the drive we fitted had failed early, or “slow” meant something different this time. The customer assumed the first, which was fair — and it was the assumption we had to test first, because the part under suspicion was ours.
Sound like your problem?
Bring it to our Putney workshop or message us a photo and we will give you a straight answer and a quote before any work starts.
Our Diagnosis
There is an order of enquiry we hold to whenever a machine comes back after we have worked on it, and it is less a technical rule than an ethical one: our own work goes under the microscope first.
If we fitted a drive six months ago and the machine is slow again, that drive is the first candidate rather than the last, even when it is not the likeliest explanation. Starting elsewhere and only reaching our own part after the customer has funded a few other avenues is how repair shops lose people.
So the drive came out and was replaced under warranty, at no charge. Our workmanship warranty on most repairs runs for 90 days; the drive itself carries its own manufacturer warranty, measured in years rather than months, so the part was covered comfortably. Accidental damage is never covered under either — a warranty covers the part and the work, not a later accident. We noted the position on the job: if the same symptom came back afterwards, that would point at a hardware fault elsewhere in the machine rather than at storage, and looking would carry no further charge.
Now the part most write-ups would quietly leave out. Changing the drive did not settle it.
What came together on the bench was not failing storage at all, but a corrupted macOS installation. Our own note from the day reads “seems like the OS is corrupted”, and the hedge is deliberate rather than lazy. Corruption is diagnosed by weight of evidence, not by a single test that lights up red. A machine that is uniformly slow, on storage known to be sound, with no one application to blame, is behaving like a system whose housekeeping has gone wrong. That is a strong indication, not a proof, and we did not dress it up as one.
How We Fixed It
The fix for a corrupted macOS installation is not a repair in any satisfying sense. It is a wipe and a clean reinstall: erase the volume, lay down a fresh copy of macOS, rebuild from there.
The work is routine. The permission is not, because it is irreversible — everything on the machine goes.
So nothing was erased until the backup position was established. We confirmed the customer’s data was backed up, and we put it in writing: what we had found, what the reinstall involved, and what would be lost if that backup proved less complete than assumed. Written confirmation before a destructive step is not box-ticking. It is the difference between a customer who agreed to a wipe and one who remembers agreeing to something else.
We did not erase on the assumption that a backup drive somewhere was current, and we did not start the wipe and ask afterwards. Had there been no backup, the running order changes: data comes off first, and where a drive makes that difficult, a recovery attempt becomes its own piece of work — charged only if it succeeds, and never promised in advance.
With the backup confirmed, the volume was erased and macOS installed clean onto the replacement drive.
The Result
The iMac went home on a fresh drive with a clean operating system, running as it ought to. The drive cost the customer nothing, because it was our part and under warranty.
It went home with a caveat, and the caveat matters as much as the fix. A clean install removes the corruption; it does not explain it. We know what the machine is doing now. We do not know for certain what damaged the installation in the first place, and there is no honest way to pretend otherwise.
So the handover carried the same message as our notes: if the slowness returns on a clean system sitting on a healthy drive, storage has been ruled out twice and the enquiry moves deeper — memory, or the logic board. The customer left knowing that is the next step rather than a nasty surprise.
Why This Happens
For most slow Macs, storage genuinely is the answer. An iMac still booting from a spinning hard drive or a Fusion Drive spends its working life waiting on that disk, and fitting an SSD is the largest single improvement available to one — the standard fix because it is usually the right one.
The trap is the second time round. Once a machine already has healthy solid-state storage, “slow” stops meaning storage — but the memory of how well the first fix worked is strong, and the instinct is to reach for it again. A second SSD is not a second fix.
macOS installations become corrupted for a handful of unremarkable reasons: an update interrupted partway through, repeated forced power-offs, a boot volume run down to no free space, software with deep system hooks removed badly, or a filesystem that was already inconsistent while it was being written to. That last one is worth a mention, though we cannot say it is what happened here. Upgraded drives are very often cloned from the old disk rather than installed fresh, because that is what most people want: the machine back exactly as they left it. A clone is faithful, which means it reproduces existing problems as accurately as it reproduces the files.
The 27-inch iMac adds a consideration a desktop tower does not. It is a sealed all-in-one, and on the slim models the display panel is bonded to the chassis with adhesive that has to be cut through and replaced every time it is opened. Getting to the internals is real work — not a reason to avoid the repair, because these machines are well worth keeping, but a strong reason to be sure of the diagnosis before opening one.
When Replacing a Part Isn’t the Fix
Swapping a suspected component is a legitimate diagnostic move. It is quick, conclusive in one direction, and where the part is one we supplied it costs the customer nothing.
But eliminating a variable is not the same as finding a fault, and the two get conflated constantly. If the symptom vanishes after a swap, the swap was the fix. If it does not, all that has been achieved is the removal of one candidate — useful, sometimes essential, but not a repair, and not something to sell as one. The honest accounting here is that we carried out work which turned out not to be the answer. That is precisely why it was no charge.
Local Help in Putney SW15
If a Mac has gone slow again after an upgrade that was meant to solve it, do not assume the answer is more hardware. Slowness on a machine that already has good storage means something different from slowness on one that never had any, and it is worth establishing which before parts are bought.
Our workshop is on Lower Richmond Road in Putney, so a 27-inch iMac is a manageable trip for anyone local — and if the problem might be software rather than hardware, a visit or a remote session can sometimes save the journey. Call 020 7610 0500 or book in for Mac repair in Putney, and do tell us about any previous repairs, including ones we carried out ourselves. A diagnostic fee applies and comes off the bill if you go ahead.
