Upgrading my imac A2116 2019

Maybe something like

• Other World Computing – Envoy
• Crucial X9 Pro for Mac (for sale by B&H Photo Video, among other retailers)
• Samsung T7 Portable SSD

You would want to plug these into one of the USB-C / Thunderbolt ports on your iMac, as those ports offer USB 3.1 Gen 2. (Your USB-A ports only support USB 3.0.)

You can spend more on a Thunderbolt 3 SSD – but it is not clear if the extra cost would be worth it for an old Mac. (Even a USB 3.0 / SATA SSD that was slow by the standards of the three listed above would run rings around your current mechanical hard drive.)

With the Crucial X9 Pros, I would recommend getting the "for Mac" versions, even if they cost a bit more. There is a recent thread on these forums claiming that the non-Mac versions might need to have firmware updated – using a tool that runs only on Windows – to make them suitable for use as Mac startup drives.

That would be an iMac (Retina 4K, 21.5-inch, 2019).

Tearing down and reassembling one of those is not a job for the faint of heart, and those Macs offer several ways to attach external SSDs, e.g.

• USB 3.0
• USB-C (USB 3.1 Gen 2)
• Thunderbolt 3

Attaching an external SSD and making it your startup drive would be the preferred way to go – if the machine allows you to do it. The issue might be that if the internal drive has already started to fail and is throwing errors, that might crash or slow down the machine even when you were started up off an external SSD, and trying to avoid any use of the internal drive.