You can revert to the old installation like described in the KB article you quoted. So there is no need to rip your RAID apart.
Like advised before download the HP customized ESXi 5.1 Offline Bundle from here: https://my.vmware.com/group/vmware/details?downloadGroup=HP-ESXI-5.1.0-GA-10SEP2012&productId=285#
Upload it on a datastore and do the upgrade from an ESXi shell with
esxcli software vib install -d /vmfs/volumes/.../VMware-ESXi-5.1.0-799733-HP-5.34.23.zip
- Andreas