Ventura usb drive won't boot and cannot be accessed

I have a MacBook Pro 13-inch, 2020.  Intel Core i5 running Catalina 10.15.7

I recently installed Ventura on an external drive. 

I have been running it ok for about a month but now when I hold down the option key to boot via the USB drive, instead of seeing ‘Ventura’ as a bootable drive I see a drive labeled ‘Ventura - data’. 

‘Ventura - data’ does not boot but leads to the Prohibitory symbol and underneath it says ‘support.apple/Mac/startup’ 

My question are

• What happened and how can I restore my original boot drive to a working state? Why has the boot drive changed name?
• How can I retrieve the files (created and edited since) Ventura was installed - given it does not even mount when I run Catalina  

Even when Ventura was booting up fine, I was only ever able to access the volume by using the hold down option key boot process. 

If I booted Catalina and plugged in the external USB drive, all I ever saw was a drive called ‘Update’ - which is still the case. (Normally, I can access any OS boot drive and the files)

If I check Disk Utility now there is a ‘Container disk 3’ and within that 3 volumes:

1. ‘disk3s4’  - which remains unmounted & is not available to access from finder window>Locations 
2. ‘Ventura - data’  - not available to access from finder window>Locations 
3. ‘Update' - available to access from finder window>Locations 

 Any guidance appreciated