Squeeze assembles one RAID array, at boot but not the other

[Hendrik Boom <hendrik@topoi.pooq.com>]

I have two RAID arrays on my Debian squeeze system.  The old one, which 
still works, and has worked for years, is on a pair of partitions on two 
750GB disks.  THe new one is not recognized at boot.

boot is *not* on any of these RAIDs; my system boots properly. 

The new one, whih I build today, resides on similar (but larger) 
partitions on two 3TB disks.  I partitioned these drives today, using 
gparted for gpt partitioning, then created a RAID1 from two 2.3GB 
partitions o these disks, set up LVM2 on the RAID drive, created an LVM 
partition, put an ext4 file system on it and filled it with lots of 
data.  The partition definitely exists. 

But it is not recognized at boot.  The dmesg output tells me all about 
finding the old RAID, but it doesn't even notice the new one, not even to 
complain about it.

It seems the significant differences bwtween the two RAIDS are:

One is new, and the other os old.
One is on a GPT-partitioned disk and the other uses the MBR partion table.
One is huge and the other is just large.

Any ideas where to look?  Or how to work around the problem?

-- hendrik