Thank you, MartinR, for taking the time and trouble to take a look. 🙂
I have already done as you suggest! Easy to miss, eh?
disk3 - LaCie Mobile SSD 1.00 TB
External USB Up to 10 Gb/s USB
disk3s1 - EFI (MS-DOS FAT32) [EFI] 210 MB
disk3s2 [APFS Container] 1000.00 GB
disk4 [APFS Virtual drive] 1000.00 GB (Shared by 6 volumes)
disk4s1 - E*********** - Data (APFS) (87.54 GB used)
disk4s2 - Preboot (APFS) [APFS Preboot] (2.11 GB used)
disk4s3 - Recovery (APFS) [Recovery] (1.15 GB used)
disk4s4 - M****************) (APFS) (9.35 GB used)
disk4s5 - Update (APFS) (2 MB used)
disk4s6 - VM (APFS) [APFS VM] (1 MB used)
I've set up this SSD ready for when my Fusion Drive fails (as it most surely will!) and it's currently available for me to boot from if I wish. I do so from time-to-time to keep the Ventura operating system up-to-date. It appears to work well.
I can, of course, still access all of my important data as it is stored in the Apple iCloud. A great boon to modern computing! 😄