Not that i'm that much of an AIX wizard, but it sounds like it tries to boot from an external SSA disk cabinet which isn't there (or has a problem), or some part in the route is defective (disk? controller?)
So far i've found people with that error code that had a bad disk, or had to setup a disk array on their RAID controller, even someone who claims his problem went away when he changed the keyboard.
"21A00xxx SCSI DASD device errors
1. Before replacing any system
components, ensure that the:
a. Controller and each device on
the SCSI bus is assigned a
unique SCSI ID.
b. SCSI bus is properly terminated.
c. SCSI signal and power cables
are securely connected and not
2. The location code information is
required to identify the ID of SCSI
device failures, as well as to indicate
the location of the controller to
which the device is attached.
3. Check below for the xxx value. The
xxx can stand for anything like 001,
002, 003 and so on. Ensure that all
components associated with the
suspected SCSI device are
21A00001 Test Unit Ready Failed - hardware error. Refer to the notes
in error code
1. Replace the SCSI device.
2. Replace the SCSI cable.
3. Replace the SCSI controller."
-= I reject reality, and substitute my own =-
1 Indigo R3k-33 32MB XS24-Z;
1 Indy R5k-180 256MB XZ;
1 Indy R4k-175 64MB XL;
2 Indigo2 R10k-195 512MB MaxImpact;
2 Indigo2 R4k-200 256MB (XL+Extreme);
2 Octane Dual R12K-300 1024MB (MXI+V6).