Server will not boot up when all drives are assigned drive letters

[MARK_MARSHALL]

We are running a 2 x DL380 G5 servers, both with attached storage (2 x msa60's on each, each controlled by a p800 Array Controller)

This is being used for our Exchange environment. This is a CCR setup with active and passive Servers.

Problem: Only on one of the servers:- If all arrays (drives) are assigned drive letters the server doesn't boot up, if 1 of the drives is not assigned a drive letter [doesnt matter which one) the server does boot up.

Can anyone help with this, as we need this server to boot up normally