Compatibility for external SSD on iMac Retina 21.5 2017 running Ventura 13.7.8

Any drives listed on the Apple website are compatible with all Macs.

Most, if not all, external consumer drives are compatible with the macOS. Most are formatted for use with a PC, but all can be reformatted for use on the Mac. Reformatting is a process that takes about a minute using Disk Utility.

Buy from Apple or find your own external drive elsewhere, it really doesn't matter. For your 2017 iMac, find something that runs at Thunderbolt 2 ($) or USB 3 speeds and you're golden.

That machine has four USB-A (USB 3.0) ports and two USB-C (USB 3.1 Gen 2, Thunderbolt 3, DisplayPort) ports.

This gives you a wide range of choices in external SSDs:

• USB 3.0 / SATA (cheapest, slowest)
• USB 3.1 Gen 2 / NVMe (probably the sweet spot in the market). These should be connected to your USB-C / Thunderbolt 3 ports, since your USB-A ports would limit transfers to USB 3.0 speeds.
• Thunderbolt 3 / NVMe (fastest, most expensive)

You can buy pre-assembled drives, or buy enclosures and internal SSDs and put the two together.

Your iMac does not support USB4 and is not compatible with USB4-only SSDs. (There are some USB4 SSDs that can fall back to other connection methods like USB 3.1 Gen 2. Those would be compatible, but their speed would be limited by whatever they and your computer both supported.)

As far as formatting goes,

• You should make sure that the SSD uses the GUID partition scheme.
• You should make sure that the SSD is formatted using APFS.

Many external SSDs come pre-formatted with NTFS (for use on Windows systems), and with them, a trip to Disk Utility to change the partitioning and formatting (wiping out everything on the new drive) should fix things so you can then proceed with putting Ventura and your data onto them.

Note: If you buy one of the small, credit-card-sized Crucial X9 Pro SSDs, make sure that you get the "for Mac" version. The "PC" version may need a firmware update before you use it as a Mac startup disk, and (Catch 22) Crucial only provides the firmware updater for use on Windows PCs.

Those external SSDs that Jack-19 recommended are the ones that Apple was selling at that time. There are literally 100's of good SSD choices and 3 connection options to choose from.

Your 2017 21.5" iMac supports USB-A 3.0, USB-C 3.1 and Thunderbolt 3 external drives.

iMac (Retina 4K, 21.5-inch, 2017) - Technical Specifications - Apple Support

A USB-A 3.0 2.5" SATA SSD on one of your iMac's USB-A 3.0 ports is the slowest option, but it is still 4 or 5 times faster than your iMac's current internal drive.

A USB-C 3.1 NVMe SSD on your iMac's USB-C 3.1 port is best option for price and speed at more than 10 times faster the your iMac's current internal drive.

A Thunderbolt 3/4 NVMe SSD on your iMac's Thunderbolt 3 port is the fastest and most expensive option.

You're welcome.