Subnostr

Check the SSD partition boot flag?

Check the bios boot sequence?

The second is good. SD, if present, otherwise USB.

How do I check/change the boot flag?

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I assume Raspberry Pi Imager should set that, no?

Yeah, not necessarily, because I guess 99.9% folks just want a portable storage device, but don't want to boot from it.

Use something like the "Disks" app or "gparted" or other partition manager. Check under "System Tools".

This is headless, so not an option, but thanks.

Looks like Bogi has the good link.

That’s all the standard stuff, which I’ve done.

dmesg?

/var/log/messages?

lsusb?