Hi Carsten
I’m helping Elleni here with this (on phone - speaking swiss german is easier for him in this stress moment…). Spending the weekend with two stints at 16 hours to prepare for going live, and getting this at the end - all with new hardware! Very frustating!
Besides which, I helped him three months ago, setting up his external “hosted” Proxmox, so we know each other also on phone… 
Unfortunately, this RAID1 mirror is the boot device AND VM Storage. And there are NO backups, as everything was migrated this weekend, including live data from the old system.
It seems that the used USB Disk - intended for a one off Backup - had fried a new laptops mainboard earlier, and maybe the Proxmox (Also new hardware) could also have mainboard issues (now).
Elleni was able to organise a (smaller) SATA Disk, and is now installing a seperate Proxmox.
We intended to try what you suggested (Had the same idea earlier…), import it in as a non-boot pool.
Thanks for any suggestions - ZFS is one of the things I don’t consider myself an expert on (yet)!
Still earning my spores…
Andy