2013 iMac not booting off USB SSD clone

My 2013 27" on Catalina 10.15 is running out of disk space. The internal 3TB HDD is broken and I have kept it working by splitting the fusion drive and running it off the still functioning but very small internal128GB SSD. In order to download a new OS and create an installer, I decided to use Carbon Copy Cloner to clone the install onto a new 480GB SATA SSD, connected via a USB to SATA adapter. The CCC process went smoothly and it created a clone in about 25 minutes. When I try to boot from from the clone, the SSD is recognised at the boot picker stage (holding down the ALT/OPT key on boot) and it begins to boot but when the progress bar has gone about 3/4 of the way across the iMac just shuts down. I have used CCC for years and always found it super reliable. Has anyone got any tips on how to troubleshoot this one?