Multiboot Grub2 edit needs saving.

My main system at this time is Devuan Jessie. I have Windows 10 on another hard disk. I have just installed Salix OS 14.2, likewise on its own hard disk. When I ran update-grub from my Devuan system, Grub2 picked up the new system as being on /dev/sdd1, however Grub2 finds the install on /dev/sdc1 at boot time. I have confirmed this by editing the Grub2 menu at boot. Despite having run a number of searches, I can't seem to find anything on how to edit and save Grub2 boot changes with a multiboot system.

You need to use UUIDs or labels, not device names. With a multi-disk setup, names can change at random depending on the order in which the newly-booted kernel finds things. The old-style kernels that used hd[a-d] nomenclature didn't have that problem as they used geometry.

An alternative solution is to have a udev rules file that enforces consistent names for drives. A lot of distros do that automatically. I'm surprised Devuan doesn't, given its Debian heritage.

PS: on rereading the post, perhaps it's Salix and not Devuan that has the numbering problem.

Sounds suspiciously like a situation I had with a mix of (E)IDE and SATA disks - is this the case ?.

Very unlikely grub cares one little bit - the issue is the init system enumerating the disks; you see the partition as variously sdc1 or sdd1 because Linux names them that way. You might be able to enforce it with udev - I seem to remember trying but never got it to work well. Might be better to use gpt and partition (rather than filesystem) UUID - never tested.