Partition of ext raid lost on reboot

An IBM Mpro system. Has 5 scsi internal disks and a external Promise Raid Ultratrak 4000 of 320GB attached to it(all on same scsi card adaptec 29160).
The promise raid is partitioned into 2 drives- v & u. V: is primary partition, U: is a logical partition. When i restart the system (soft or hard boot)the U: drive disappears! using Disk admin i find that the drive is set to “do not assign drive letter”. on setting it back to u, i get all my data back. What could be the problem?