Booting iMac Catalina with Memory Card

[coke]

Why use an SDcard, instead of an external SSD?

 

https://www.amazon.com/Samsung-Portable-MU-PA250B-AM-Alluring/dp/B073H552FK/ref=sr_1_8?keywords=ssd&qid=1582945609&refinements=p_89%3ASamsung%2Cp_n_feature_keywords_five_browse-bin%3A7688214011&rnid=7688213011&s=pc&sr=1-8

[coke]

It's probably trying to look at both for booting. I would take out the card, reboot and make sure the internal SSD works before using the card. It really doesn't seem as thought the card is working. Eventually, you will have to have your computer checked out by an Apple support or another expert. 

[coke]

Before you spend $800 or more, I would try to take it to Apple, or an Apple Certified Macintosh Technician, to get it checked out before buying a new one.