Seagate is not allowing me to make partition on MS-DOS (FAT) Or ExFAT!
By accident I found that, by attempting to reformat my Seagate drive as FAT32 (which failed), I was then able to reformat to Mac OS (J Ex) with the partition options available… This then said it was able to work as a startup disk – not tried booting from the CCC backup yet, fingers crossed!