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root=/dev/sdXY rdinit= ro
Incidentally, I do think laprjns is probably right that you don't actually need to bother with the initrd or UUID. The fact that (if I have understood correctly) your Devuan and Salix are ordering the devices differently after boot shouldn't make any difference to booting itself, as long as you don't move the drives around again.
If you continue to have difficulty booting your Salix from the USB, it would probably be worth pursuing this option, because then all you would need to do is install the correctly configured bootloader (without UUID) from Devuan. You could forget about both the USB booting and the initrd.
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If I boot from Devuan, the target disk is /dev/sdd