Problems with twa

[Giulio Ferro]

I've installed a 3ware 9500 sata controller with 4 1TB disks
in raid 5.
Apart from the usual wrong geometry warning the starting configuration
is ok: I create a single partition with the default labels plus a /usr/home
label with takes most of the disk space (about 2.7TB).

When I reboot the system the /usr/home partition size drops unexplainably
to about 640GB.

In order to try to understand what happens I've installed freebsd anew, this
time creating 2 partitions: the first one 50GB, the second about 2.7TB (the
remaining space). Everything seems to work correctly.
I reboot the system and lo! the second partitions shrinks to about 640GB 
and
the partitioner (using sysinstall) tells me that there are 2048GB free!

This happens both with freebsd 7 and 8 current amd64.

Thanks for any help.