Hi dep,
So, then, its spare sectors come from unpartitioned, unformatted space?
Not all of them. The drive itself has, when it leaves the factory, a bunch of spare sectors *beyond* the advertised 500GB. And these can only be accessed by the drive-firmware, nothing else. So when you leave mentioned 25GB unpartitioned, you have the equivalent of 25Gb plus a bunch extra cells! as spare.
My plan is to put everything except /home there.
It's up to you to leave the gain in speed for that unharvested.
20.04-LTS I no longer have a grub menu. Might this be because I no longer have a dual-boot system? (I nuked the XP partition because I hadn't booted to it in years.)
Run update-grub.
which is whether a drive's physical location in the SATA system mattered in assigning drive designations. If I read you accurately, by using UUIDs this wouldn't matter
Nik mentioned a case, where this wouldn't work as expected. But Linux prefers to use UUID (Universally Unique Identifier), LABEL, or symlinks to identify media storage devices on a system. Using device assignments (like /dev/hd*# or /dev/sd*#) is not preferred since these can change between system boots. Source ubuntu documentation.
Regards.