It took some time as I was busy, but now came back to this. I created a normal Ubuntu 20.04 VM and added the USB drive. Within the Ubuntu VM I can see the device as /dev/sdb with its only partition beeing /dev/sdb1. I can mount this partition and it seems to be a correct unraid root (no great surprise, as booting from the USB drive on bare metal works as expected).
In the boot manager there is an option Boot Maintenance Manager >> Boot From File. I would expect two entries there, one for the regular disk which contains the Ubuntu boot partition, and an entry for the USB drive. But there is only one, refereing to the first harddisk, which is the Ubuntu boot drive. So I guess, the current boot process doesn’t have the USB drive ready to use. I tried to set
reuired=true for the device, but I guess this option has nothing to do with the boot process, and just avoids starting the instance without the device being present.
So I’m again out of ideas pretty fast. Actually I’m quite happy with that result too, LOL, it’s a really good point about LXD: It’s so clear and reduced about it’s options, that you don’t need to mess around for hours or even days reading cryptic, outdated and useless documentations like with other tools. Although I would be happier if it just would have worked But I’m sure, what I try to accomplish, is not yet thought to be done easy in the current state of VMs in LXD.
Is there any option left to try? Something to make the USB drive accessible during boot?