Ubuntu boots to recovery since adding extra HDD's

Hi, still pretty new to Ubuntu, so please go easy on me

I've setup a NAS server on Ubuntu 16.x in my HP Microserver

Everything working well, until I added a couple of extra drives to the 4 bays

Now, it boots into recovery mode, saying it can't find an ext4 partition on sdc

From what I can determine, Grub is expecting the install to exist on sdc, but where I've added the extra drives, by boot drive has been relegated to sde
(If I take the new drives out, I can boot to the install no problem)

From what I've read, I think that I need to 'install' grub to sde - but first need to get recovery to boot to the system (as of course sde doesn't exist when I take the drives out)

So, within Recovery, I've install grub, and I'm following some 'recovery' steps...

grub> set root=(hd4,1)
grub> set prefix=(hd4,1)/boot/grub
grub> insmod normal
grub> normal

Hope that all makes sense!