I've resolved the issue of hdb not getting detected, and have created a raid 0.90 linear array, out of the 4 cards. ( only for the music partition). I've symlinked /dev/hda4 to /dev/md0 so the player sees it all as one big disk.

Currently rsync'ing my music.

Once the rsync has finished i'll report back on boot times etc.

I stopped the rsync and rebuilt the array as a raid0 so that the 4 cards are accessed in parallel, so that if the rsync completes ok i can be more confident that the system is stable.

The rsync complete successfully.

The player boots up on AC in 15 seconds compared to 21 seconds for my hard disk based player.

A couple of those 15 seconds are spent assembling the RAID0 array, When I was using a LINEAR array it was a little quicker.