Booting from RAID

[Amias Channer]

Hello all ,

I have heard people mentioning that running the OS on a RAID device is a
bad idea , why is this ?
We have an external SCSI raid controller that presents itself to the
system as a SCSI hard drive (/dev/sda)
Would i be right in assuming that putting the OS on this is not a
problem as the RAID functionality 
is not controlled by the computer and that it would only be a problem if
the RAID was controlled by the OS ?

Thanks in Advance

Toodle-pip
Amias