Although you may know there is lots of room left on the hard
disk, it is not unusual to run out of space on one of the disk's file
systems or partitions, in which case an error message like
NO SPACE ON DEV 0,1
will appear on the console, and
your program or command may fail with no further ado.
Likewise, if one of your programs produces an error
message when it tries to write a file and you just know you never had
that trouble before, it may well be that you have run out of disk
space rather than into a programming error. You determine your
personal disk usage, as well as that of the entire system, with the
commands du and df.