|A bit more information is required:|
Is it 'suddenly' disappearing, but was ok before ?
Is it a newly connected device ?
Is it the only device on it's SCSI bus/interface ?
If no, are the other devices on the same bus ok ?
The problem can be the interface, the cabling, the drive itself.
When checking cabling, powerdown etc., don't wait for VMS booted,
use the Alpha console (SRM):
>>>init (after any change in connection or powerstate)