The UNXSIGNAL bug check has been observed on versions of VMS prior to OpenVMS 7.3 when attempting to write to a new disk.

**** OpenVMS (TM) Alpha Operating System V6.2     - BUGCHECK ****
** Code=0000041C: UNXSIGNAL, Unexpected signal name in ACP
** Crash CPU: 00    Primary CPU: 00    Active CPUs: 00000001
** Current Process = BPCD_BG30
** Image Name = BUILD$DKC0:[OPENV.NETBACKUP]BPCD_AXP.EXE;1

This crash is due to an old disk geometry problem.  The problem causes an overflow in one of the following UCB fields:

UCB$B_SECTORS
UCB$B_TRACKS
UCB$W_CYLINDERS  

This results in the VCB$L_BLOCKFACT field in the VCB being zero.  The first space allocation on the volume generates a divide-by-zero error, which crashes the system with a UNXSIGNAL bugcheck.

The fix is in MOUNTSHR.EXE in V7.3 (VAX and Alpha).

Fixes are also available in the following patch kits:

MOUNTSHR.EXE_V62R
VMOUNT.EXE_V62R
VMS73_F11X-V0200
VMS73_F11X-V0300

The workaround is to change the size of the virtual disk.  For example, if an RZ25 generates the bug check, change the disk to an RZ56.