just another point of view, I do not claim it is right
I think you can have grub as bootloader on a raid1 no problem.
Sometimes the install procedure is "strange" on raid1 for some distro,
requiring to install on 1 drive, then duplicating on the second
split between 4/2 cards: I would look at
and see which card is best supported http://linuxmafia.com/faq/Hardware/sata.html
In case of sata card "breaking" I would have rather 2 HDs on each card.
Other have your life ""simple"" solution:
install linux on the IDE and have a dd backup (crontab at regular interval)
if one HD packs up in the raid, if they are same model, the risk is
increasing both HD may have been manufactured round same time
and could pack up as well
What's the ventilation like for these 5 HDs
NB: Any linux distro is very roomy on 10 Gb