Hello,
For stability reasons I’m trying to set up a sytem booting RaspberryPi4 from a an USB-stick and having NextCloud Data on a RAID system. Up to now I failed, as NCP cannot identify/work with 2 USB3 media. A standard seutup with SD-card an RAID-system worked.
How can I manage using USB-stick for boot and RAID for storage.
Thanks for Your help
Hi there 
My setup is using the SD card as boot only with my root on an external hdd in it’s own partition with the NCPi data on another partition, I could be wrong about this since I haven’t tried getting RAID to work myself however I did read a little about it as I was installing it on my system.
It seems like RAID support in the EEPROM is not supported and you need to manually create the RAID as well as set the PARTUUID / UUID identifiers for each of your drives in the /etc/fstab to configure it for RAID use among some other settings as well.
Here is a link to the Raspberry Ṕi forums where a user made a post about getting the RPi4 to work in USB RAID, I think something like this is what needs to be done for your configuration to work properly.
EDIT: Note as they are saying in the Raspberry Pi forums that since RAID is not supported in the Raspbian Linux Kernel you will run a high risk of losing your data and backups in such a setup is crucial beyond belief.