Am setting up a TrueNAS Scale system (Aoostar R7/64GB RAM/2x2TB NVMW/2X12TBHDD) and booting from a 256GB M.2 NVME in a USB enclosure. The system has a MicroSD Card slot.
Any problems/issues using it as a place to backup the boot drive?
Is it a problem to not have a mirror on the boot drive?
I don’t see a reason personally to back up the boot device on TrueNAS. It’s more important to keep backups of your configuration file.
That said we expect the boot device to have not terrible write endurance and performance because we write audit entries and logs to it (something to bear in mind when choosing hardware).
The decision as to whether to mirror your boot devices is more of an issue of how much downtime you can tolerate while replacing failed boot medium than a functional requirement for TrueNAS. Compared to potentially other issues with the build this is maybe small-potatoes.
That said, if reliability is a high priority for you you should use server-grade hardware.
It’s a few KiB. Essentially an sqlite database. Generally, it’s a better idea to store it on a separate computer from the NAS (and in multiple locations).