hi which ssd can i use to upgrade my 2017 21.5in imac pls? the non retina model mid 2017

I think that most of us here would suggest using an external SSD.These days they are reasonably priced and very easy to install using a USB cable. All you might have to do is to reformat it. Then you can install your operating system on it and use it as your start up drive. Simple and easy to do.

100 percent agree with Ronsara 👍🏼 with this caveat: Not "just any" external will work. The enclosure must be rated USB 3, and the SSD inside must be rated at 6GB/sec SATA transfer speed.

Having suffered at the hands of most 'name-brand" external drives, I now buy only OWC Mercury drives for all our Macs. Balancing performance and cost, I'd go for this one:

https://eshop.macsales.com/shop/owc-mercury-on-the-go-pro/usb-3.1

with the optional external power supply:

https://eshop.macsales.com/item/OWC/MOTGPWR/

I recommend the power supply because this is going to be your boot partition and you don't want a low-power episode from a USB port causing any hiccups.

Remember, you do not need an external SSD boot volume to be the same size as your current internal. You will still have access to your internal drive for data storage.

The good thing about this solution is , should your replace the 2017 model, you still have a working drive that will serve on a newer computer.

100% agree with an external solution OWC solution.