systemd.unit=rescue.target

Just another quick one liner: a Grub config argument which I had to dig for but which is really useful when this sort of thing happens

Faced with a server that was rebooting after an upgrade and dropping to sytemd emergency target:

Rebooting and adding

ssytemd.unit=rescue.target

to the end of the Linux command line in the Grub config as the machine booted and then pressing F10 allowed me to drop to a full featured rescue environment with read/write access to the disk and to sort out the partial upgrade mess.