Installing and running macOS on external SSD to replace HDD from 2017 iMac

Hi,


I am looking to install an external SSD on my older iMac (2017) to replace the existing HDD for running MacOS (Ventura 13.7.8). My current HDD is running slow and I have been advised that installing an external SSD instead can speed things up.


My question is what format to use for formatting the SSD in order to make it work with the existing HDD?


Any help is appreciated. Thanks

iMac 21.5″ 4K, macOS 13.7

Posted on Oct 25, 2025 7:43 AM

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Posted on Oct 25, 2025 8:42 AM

Jack-19 created an awesome User Tip just for that.

see > Use an external SSD as your startup disk … - Apple Community


The answer to your formatting question, is down in section 3.

5 replies

Oct 25, 2025 10:28 AM in response to fcars

+2 on Jack-19's instructions. Also, please note a 2017 is now an older machine that is considered obsolete. This means (as you likely already know) it cannot run current versions of Mac OS and parts are not readily available. If the machine is your primary computer, you may want to consider replacing it rather than spending money on an old computer. Yes this will be more $ that just buying an external high quality SSD however there are many many more benefits you will have. Just food for thought.

Installing and running macOS on external SSD to replace HDD from 2017 iMac

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