Philip,
The number displayed by SHOW DEVICE as the free blocks on the disk is just a number, and it can drift. It plays no part in allocation, so if you attempt to allocate space, and there is space available, the allocation will work regardless of what SHOW DEVICE says.
$ SET VOLUME/REBUILD
should bring it back into line, if that fails, try
$ SET VOLUME/REBUILD=FORCE
If that still doesn't work, you may have "lost" files, which can be recovered with ANALYZE/DISK/REPAIR.
Note that DIRECTORY doesn't necessarily give an accurate result for disk consumption (try it on a system disk!). It can over or under estimate, depending on how you phrase the command. If you are counting up files, make sure you use the allocated size DIRECTORY/SIZE=ALL or F$FILE item ALQ.
Also note that the concept of "number of free blocks on a disk", is not as simple as it might first appear. There are legitimate reasons why multiple nodes in a cluster might correctly report different values for free space, and, in a multi user environment, the values can vary wildly from instant to instant. You should therefore avoid code with logic like:
IF sufficient-disk-space THEN
do something
ELSE
handle error
ENDIF
because it will suffer from both false positives and false negatives due to the finite time between sampling and acting. Instead it should be coded as:
do something
IF error
THEN
handle error
ENDIF
A crucible of informative mistakes
