My late 2015 desktop IMac has become unusable - Advice badly needed !

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Level 1

12 points

My late 2015 desktop IMac has become unusable - Advice badly needed !

Because I have been unable to upgrade the Apple Operating System beyond Catalina my late 2013 Imac has become progressively slower & slower as it seems the Catalina operating system software I depended on for normal day to day functionality seemed to progressively 'disintegrate' & fail to work at all in many instances.

Just now for instance I successfully changed the password on a user account in order to access it as I had forgotten the original password. It did allow me that access to the user account but then unaccountably it just started building a brand new user account and I could not stop that process.

I then decided to delete all the user accounts but could find no way of doing that so clicked on "help' to get the usual help document normally available which provides detailed instructions on how to use Apple software.

I just recieved a message saying that help document was no longer available at all. So I then telephoned apple support and the Apple Support person advising me advised me my computer might not even re-start all if I turned it off to re-boot. And he advised me that even if I did a full back up of the computer contents and then put that backup onto a brand new imac with the latest operating system it would be unlikely that newer imac would be able to read all the older documents that I have already been barred from accessing on the grounds the operating system keeps saying you cannot open that particular document as you do not have the correct software to be able to open it. etc.

This is just part of the complete chaos that has now infected my imac because I cannot update the Catalina operating system to the latest operating system because Apple have frequently told me software is designed not to function on older machines and that I cannnot reasonably expect to be able to use any hardware which is more than six years old because advances in 'technology' make previous hardware 'obsolete' and non-functional.

As I have to wait another couple of months or so to be able to accummulate the £2000 I need to buy a new apple desktop & even then have already been told by Apple I still won't be able to access all my older documents I have no idea how to sort this problem out at all !

Has anyone else worked out how to to deal with this astonishing situation ? Any advcie would be most welcome.

Posted on Oct 29, 2025 3:25 PM

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Answer 1

HWTech

Level 9

68,180 points

Oct 29, 2025 6:16 PM in response to Ziggyzig1

Performance degradation for such an old computer is most likely due to a Hard Drive failure. The other most common cause is an issue with third party software especially anti-virus apps, cleaning/optimizer apps, and third party security software.

You can try running the third party app DriveDx (free trial period) to check the health of the Hard Drive. Post the complete DriveDx text report using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar.

To look for other issues, run the third party app EtreCheck and post the complete report here so we can examine it for clues.

How to use the Add Text Feature When Posting an EtreCheck Report - Apple Community

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Answer 2

Old Toad

Level 10

215,082 points

Oct 30, 2025 11:24 AM in response to Ziggyzig1

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Do you have a Fusion drive or and HHD?

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Answer 3

Oct 29, 2025 3:59 PM in response to Ziggyzig1

The iMac (Retina 5K, 27-inch, Late 2015) shipped with OS X 10.11 or 10.11.1 (El Capitan) – and you can upgrade it as far as macOS 12 (Monterey). So it received a total of six major macOS upgrades.

Neither Catalina nor Monterey is among the "most recent three" supported by vendors like Microsoft and Adobe … and neither gets the most recent version of Safari. But Monterey is a couple of versions more recent than Catalina, and should have a few more features, bug fixes, and security fixes.

There are a few applications that might let you get more use out of that Mac. As far as I know, the current versions of

The Firefox Web browser
The LibreOffice office suite
The GNU Image Manipulation Program (GIMP)
The GraphicConverter photo editor (this one costs money)

will run on Catalina. Most other major third-party Web browsers (Brave, Opera, Chrome, Microsoft Edge) will run on Monterey.

You can get Monterey using the App Store link in this Support article.

How to download and install macOS - Apple Support

However, before you do that, I think it may be useful to try to troubleshoot why your Mac has gotten so slow, and to make sure that you have current backups. There may be a hardware issue (such as a failing hard drive), or perhaps there is some sort of software (like antivirus/security software or "cleaning" software), installed recently, that could be eating up resources or interfering with system operation.

As far as "I then decided to delete all the user accounts but could find no way of doing that", that is a Good Thing. You do not want to delete all of the accounts, or all of the Admin accounts. You need at least one user account to use the computer, and one Admin account (which can be the same one) to be able to manage it.

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Answer 4

Allan Jones

Level 9

77,588 points

Oct 30, 2025 12:03 PM in response to Ziggyzig1

So is this a 2015 iMac as shown in the title, or a 2013 as in your first line of text? Could make a difference to getting accurate help here where we can neither see nor remotely access your iMac.

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