Simple Trick - Revitalise that old iMac


Feb 13, 2012
Gigabyte Z390M Gaming
RX 570
iMac, Mac mini
Classic Mac
eMac, iBook
Mobile Phone
As well as Hackintoshing I also run a couple of old real Macs, an iMac from 2015 and a Mac mini from 2014.

These are pretty slow nowadays having old-fashioned laptop HDDs spinning at only 5400RPM. This makes boot-up and scrolling a round in Finder a very sluggish pass-time. Constant spinny blue or rainbow beach-balls. Apps slow to load and react to changes.

With the Mac Mini it is relatively easy to take it apart and replace the HDD with a new SSD, but the iMac is more complicated and involves removing the LCD to get at the internals, and then sticking it back when done. You may not want to risk doing any damage, or feel confident enough to do the work.

Well there is an easier way that gives that old machine an instant performance boost and won't damage anything in the process.

You can fit a new SSD in an external USB caddy or enclosure and connect it via a USB cable to your machine and boot from there, bypassing the old HDD you have inside.

At first I thought there must be a hitch, that it wouldn't work very well and at best it would only speed-up a small proportion of what I do. I was wrong. This process improved the whole user experience for my old iMac.

This is only going to work well if your Mac has USB3.0 ports. USB2.0 will work, it's true, but the results aren't going to be so good.

Step one, you need a good quality external caddy to put the SATA SSD you chose, into. I picked a UGreen USB-C USB3.1 Gen 2 box for the fastest possible transfer speeds. (Even though my old iMac's ports are slightly slower, it doesn't hurt).

To go in it you can use any SATA SSD you like but bear in mind it doesn't need to be the fastest because the USB3.0 port is our bottleneck. For this test I used a budget Crucial BX500 120GB. The difference between this model and the MX200 range is warranty and read/write speed. But for around $20/£20 I intend buying another when Big Sur finally arrives anyway:


Step two is to put your new SATA SSD into the caddy and plug it into your iMac. When it appears on the Desktop use Disk Utility to partition it as Apple Extended Journaled, with the GUID scheme. Give it a name you'll recognise.

Step three is to install macOS. My old iMac had been reverted to High Sierra to save it's embarrassment at being slower than molasses most of the time. I knew from bitter experience that Catalina was appallingly slow on it, but I still wanted to get there in preparation for Big Sur later this year. I downloaded the latest installer from the App Store and then, rather than just running it from the Application folder, instead created a USB installer using the Terminal "createinstallmedia" as per Apple's support web-site. This took around 20-minutes to complete.

Once I had this, I booted using the Options key so I could choose which OS to boot from. I selected the USB stick (which Apple had labelled "EFI Boot" but is also orange as USB devices are) and the Catalina installer began. Once it was running and asked where to install I pointed it to my external SSD and let it go to work.

Another 20-minutes or so and we had a Catalina desktop. (A tip here is to keep an ear out for any reboots and use that Option key again to ensure the correct drive is chosen. I forgot to do this once and the iMac booted into my BootCamp partition instead.)

Next you will be asked to set up a user account and sign-in to iCloud. Personally I would not sign-in to iCloud yet, not until you are sure everything is working as you want it.

Take a look around. You will find everything is smoother, even considering the new APFS file-system Apple has installed/updated. Apps load faster and because they often access the disk, are faster too, generally. With a good quality caddy such as the aluminium one I chose, heat dissipation isn't much of a problem. The cover gets a little warm but that's all. Catalina now works as I'd hoped it would before. Way better than when on the internal HDD. A breath of fresh air to an old machine. And sitting on the iMac stand doesn't look out of place:


You can make your choice of boot volume permanent with the System Preferences - Startup Disk tab.

After this your original macOS installation can be kept as a back-up or deleted and the whole drive used as storage. Your choice. However, if you decide to go this route remember to sign-out of everything you can and de-activate any expensive software you've installed, including iTunes. Personally, I'm leaving the old High Sierra install in place, but the rest of the drive is now data storage.

I will be re-visiting this article to keep it updated. Any feedback is welcome.

Parts used:


External enclosure -

Last edited:


Jul 31, 2012
HD4600 / RX 570
iMac, Mac mini
Mobile Phone