USB-boot of fully configured NCP not working after test installation of another USB device

I’m running NCP on a Raspi 4B 8GB from a 480GB USB-SSD using 3 Partitions (/boot (fat32), rootfs (ext4), /media/data (btrfs) ) which worked fine until I shutdown to test a new clean instance. I used a new USB-Stick and the same raspi4. I flashed the new Nextcloudpi image to the new stick and booted. I wanted to install a clean basic system and restore stettings and user-data from my backups. After installing the new sytem i shut it down and wanted to boot the old, formly well running system from the SSD which contained the old installation and was totally untouched. However the old installation did not boot. I tried with another cloned backup SSD which was not booting either.
What is wrong ? Why won’t boot the raspi 4 ?

After lot of testing i found a solution: flashed the backup-image of a early NCP-microSD-card backup (containing /boot and /rootfs , no data) onto an empty USB-drive. From this i cloned the /boot -partition using “partclone” onto my regular NCP-Backup SSD. This worked luckily…
NCP booted again and i think i should take a deeper insight into raspi 4’ boot and shutdown process.