[CentOS] sw raid issue with C5.10-xen

My raid array won't boot, gives a kernel panic - attempt to kill init message and goes into endless reboots.

MB is a supermicro 2P with opteron 2376 4-core cpus, centos sw raid, installed C5.3, updated as possible, currently c5.10. 5 seagate 750GB drives as raid-10 and hot spare. we set this up about 5 years ago and IIRC we set aside the first 10GB on drive 0 as a boot partition.

Grub runs and the system tries to boot 2.6.18-308.20.1.el5xen, sees a dirty raid array and

md: md0: raid array is not clean - starting background reconstruction

which looks hopeful but then

md10: not enough operational mirrors for raid 10

and then the panic and reboot.

ideas? stuff to read? diagnostics?

TIA,

Dave