I have a 2017 iMac 5k retina with SSD and HDD. How do I set up SSD as startup drive?

A 28GB SSD is too small to use as a start up drive.

MacOS requires about 30GB minimum depending on the version, with 60GB+ strongly recommended to leave free space for new applications and system temporary files.

I would instead suggest using an external USB-C SSD as your start up drive. Use an external storage device as a Mac startup disk - Apple Support

You likely have a split fusion drive which requires erasi the two drives to fix... do you have good current backups?

About Fusion Drive, a storage option for some Mac computers.

If you continue to have problems with your disk or it can’t be repaired, it may be physically damaged and need to be replaced. For information about servicing your Mac, see Find out how to service or repair your Mac.

In Terminal…

diskutil resetFusion

How to fix a split Fusion Drive - Apple Support

Or install to & boot from an external Thunderbolt SSD...

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That iMac has both USB-C (Thunderbolt 3) ports and USB-A (USB 3.0) ports, so you could use almost any external SSD on the market. Even a USB 3.0 / SATA one should offer an big improvement over a mechanical hard drive.

You probably would not want to spend extra to get a SSD that connects via

• USB 3.2 Gen 2x2 ("up to 20 Gbps")
• USB4 20 Gbps mode
• USB4 40 Gbps mode

as your Mac does not support those, and communication would probably fall back to USB 3.1 Gen 2 speed.

Hello.

To set up a start up disk, simply open system settings, go to start up disk, and select the disk you want it to boot it up as. Or, boot in to recovery mode and select (boot in to) the disk you want to start up your Mac as.

Thanks