Use an external SSD as your startup disk with your iMac or Mac mini

Your Retina 5K 27" 2017 iMac can startup from an external SSD as well.

If you are referring to the user tip by Jack-19 > Use an external SSD as your startup disk … - Apple Community

I can't say for sure. Jack-19 was trying to cover all of the 21.5" and earlier 27" iMac's that have slower spinning hard drives. While the Retina 5K 27" 2017 model with a Fusion Drive is not on the master list, it is mentioned in several other places of Jack's user tip.

Ok, interesting. I’ve had excellent luck with that model.

I believe you should create an EtreCheck report. EtreCheck is a simple, helpful utility to diagnose and identify both software and select hardware issues.

If your Mac is experiencing an issue, such as poor performance or running slowly, kernel panics, apps freezing or quitting unexpectedly, storage issues, and many more, generating an EtreCheck report is a great place to start for troubleshooting issues with your Mac while getting help at Apple Support Community. EtreCheck was developed by a highly respected member of these very Apple Support Community forums, and it is trusted by many users.

Downloading

To get started, download EtreCheck:

https://etrecheck.com/en/welcome.html

Click “download” on the site. The app will be downloaded to your Mac to your downloads location, which is the “Downloads” folder by default. This can be accessed by going to the Finder > Go (in the top menu bar) > Downloads.

Open the app, and generate a report

Open the downloaded application, and follow the initial steps to open the app. Choose a problem in the drop-down menu, and click Start. Creating a report typically takes 2-4 minutes on a healthy Mac.

View your report, and upload a copy to Apple Support Community

Once you have your report generated, you can upload it to Apple Support Community for us to review and provide suggestions. Please refer to this document with instructions for uploading your report and using the “Additional Text” function of Apple Support Community:

How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report - Apple Community.

If you need help in the meantime, please let us know!

Jack

I have the 2017 27" iMac18,3 model and boot regularly from external SSDs to test beta systems and run older systems for particular software. The type of drive installed in the iMac is not relevant.

That model has Thunderbolt 3 ports so make sure any external SSD you get has Thunderbolt 3 connectivity. OWC (MacSales.com) is a good place to start.

Don´t worry, tilianova, your iMac will work perfectly using an external SSD as a startup drive! I have the exact same iMac as you (iMac, Retina 5K, 27", 2017), and although I´m running Big Sur on the internal Apple SSD I often use external SSDs to boot from older versions of macOS like Mojave, for software reasons. No problems at all! Cheers, Ian.

Actually, that appears to be the hard disk, not SSD, model. Yes, a hard disk will be slow. You need an SSD.

You're welcome.

I put out a bat call to Jack-19, hopefully he will respond.

Hello - I just saw your new response.

Thank you - have now got the disk up and running, but disappointingly slow.

Are you referring to an external SSD? If so, what model did you end up using?

Jack