Existing Ubuntu Studio installation (21.10) is on sda.
Plugged in external USB disk (500G) - sdc. Wiped partition on it with gparted.
Booted into 20.04.4 Ubuntu Studio Live DVD & chose install. Chose to install alongside 21.10, and it correctly installed to sdc.
On reboot, I got the grub rescue prompt.
I had to boot back into the Live DVD, chroot into sda, reinstall grub, and upgrade grub.
After a reboot, I can now boot both installations.
Existing Ubuntu Studio installation (21.10) is on sda.
Plugged in external USB disk (500G) - sdc. Wiped partition on it with gparted.
Booted into 20.04.4 Ubuntu Studio Live DVD & chose install. Chose to install alongside 21.10, and it correctly installed to sdc.
On reboot, I got the grub rescue prompt.
I had to boot back into the Live DVD, chroot into sda, reinstall grub, and upgrade grub.
After a reboot, I can now boot both installations.