If I use the Debian Stable (Squeeze) Installer to install Debian, booting from a RAID1 pair of drives, does it write grub to the MBR of both drives - if not, how should I best protect against a drive failure - if I just clone the MBR won't it still point to the failed device?
Not sure if the installer does it by default, but if (once the system is booted) you run:
just hit enter until you're prompted for "GRUB install devices", then you can select the MBRs of both drives.
Here is a screenshot of the relevant screen in Debian Jessie (8.3), with GRUB2 version
If you install Grub legacy, then it's only installed on the one boot disk. You can add it to the second disk's MBR by running
You can install Grub2 on Debian squeeze, and I recommend that if you have any sort of nontrivial setup such as RAID. Grub2 has the advantage that you can put your whole system on LVM, you don't need a non-LVM