CAN MY 2020 iMac be run off an external SSD?

Yes, it is possible. You'll have to update the security settings to do that ("Allow booting from external or removable media"), though: About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support.

Edit: @rkaufmann87 is absolutely correct. The 2020 iMac was only offered with SSDs. Double-check the year of your Mac by clicking  > About This Mac. You probably have a 2019 model or older. In which case, it is not equipped with the T2 chip so the link above is meaningless.

On 2019 or older iMac, you won't have a security issue booting from an external drive: Install macOS on an external storage device and use it as a startup disk - Apple Support.

Return the thing. Moving forward NEVER EVER buy a used Mac from:

• Craigslist
• Ebay
• Local PC Shop
• Marketplace
• Amazon
• Friend, neighbor, relative or aquaintance

Why, simple because more often than not you have no idea what you are getting. In addition in almost all cases the computer has not been setup correctly for resale (YES THIS IS CRITICAL) and the computer is NOT what you expect, just like what you bought.

If you cannot afford an genuine Apple refurbished Mac then buy from a reputable reseller such as Macsales.com. I can assure you what you bought is not a 2020, somehow it has been modified and you just bought someone else’s hack. Look up 2020 27” iMac technical specifications and you will find what Apple published.

iMac (Retina 5K, 27-inch, 2020) - Technical Specifications - Apple Support

Where exactly did you get the machine? Apple NEVER sold any 2020 27" iMac's with a hard disk.

You're welcome. FWIW, doing a little research that appears to be a 27", Retina 5K, 2017 iMac. At least that would match the CPU & GPU. Ventura 13.7.3 is available with the latest security updates so you should update if possible. I can't say how secure a hacked iMac may be.

If that Mac is a 2017 iMac – which matches what we can see about the mechanical drive, the CPU, and the GPU – it could run Ventura without being hacked. The dialog box says that it is running Ventura now.

If it wasn't hacked for the purpose of running Sonoma or Sequoia, that might suggest that it was hacked specifically for the purpose of committing fraud against an unsuspecting buyer – making them believe they were getting a 2020 machine, so they would pay the seller more than a 2017 machine would be worth.

All 202 27” iMacs have SSDs, you don’t have a 2020 iMac! Also was it prepared correctly for sale? If you are unsure then return it ASAP!!!!!!!!!!!

FWIW, that iMac is seriously hacked. Probably so it could run a version of macOS that wasn't supported. Aside from the HDD issue, it also lists a CPU & GPU that wasn't in a 2020 27" iMac.

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