If you need to recover data off of it, please do NOT mess with the RAID facilities of the server. Don't try to reprioritize drives or move RAID volumes around. Use something safer that has no chance of modifying data or metadata.
Look for a forensic portable OS, such as DEFT. Many have undelete programs baked in. Boot off of USB or CD simply by raising USB boot to a higher priority. You might have to experiment with distributions for drivers that are compatible with what network cards and storage controllers are in your machine.
Also consider performing a full copy of the existing volume before doing anything. Perhaps a
dd to another volume. At best, a write blocker that can clone the drives (made difficult by the RAID configuration, of course. Keep the drives in order and write down exactly where they're plugged in!)
To answer your exact question:
How can I boot from this drive?
It may be that the drives you are trying to use are not supported by the newer HP storage controllers. Especially pertinent if you're using a non-HP drive, like a Western Digital or Seagate.