Using Boot Camp Assistant, Windows Setup cannot create partition or find existing one

Hello,

I've searched these forums for similar issues and some of the suggestions have been helpful, but I'm now stuck and can't find an answer.

I'm trying to run Windows 10 on my iMac, late 2013, using Boot Camp Assistant. I'm running Catalina 10.15.7 from a 1TB external SSD and I have a separate flash drive with the ISO image and the Windows support files. I understand that I cannot run Windows from the external drive so I seem to have to run Boot Camp twice to partition the HDD; the first time I run it, it partitions the SSD (there is no option to select a different drive), then it stalls after I input my password. Then I restart Boot Camp and create the partition again, and this time it partitions the HDD and reboots in Windows after inputting my password.

I get as far as the Windows Setup screen where I select the partitioned HDD called "BOOTCAMP" but get an error message stating: "We couldn't create a new partition or late an existing one. For more information, see the Setup log files."

I've tried removing the partition using Disk Utility and re-running Boot Camp. I've also tried the same but removing the partitions on both drives. I've also tried removing both partitions and then doing a PRAM reset. I still keep getting the same error. And I don't know where to check the Setup log files.

Can anyone help with a fix? Many thanks in advance.

Jon