Q: Mac Pro says startup disk is nearly full with 65 GB free

Mac pro 2012 ; 24 GB ram. 240 GB SSD boot disk. Mountain Lion, patched to current.

Boot disk contains ONLY operating system and applications.  /Users is a symbolic link to /Volumes/Users, a separate pair of mirrored drives.  The machine has

Why am I getting complaints about startup disk being full when it has 60 odd GB of free space.

$ df -H

Filesystem      Size   Used  Avail Capacity   iused     ifree %iused  Mounted on

/dev/disk0s2    119G    57G    62G    48%  13953139  15142163   48%   /

devfs           220k   220k     0B   100%       743         0  100%   /dev

/dev/disk7s2    4.0T   3.1T   883G    78% 380603328 107732004   78%   /Volumes/TickTock

/dev/disk6      3.0T   2.1T   878G    71% 259093194 107148136   71%   /Volumes/Aldus

/dev/disk5      2.0T   1.6T   433G    79% 382504771 105789883   78%   /Volumes/Zaphod Beeblebrox

map -hosts        0B     0B     0B   100%         0         0  100%   /net

map auto_home     0B     0B     0B   100%         0         0  100%   /home

/dev/disk8s2    3.0T   2.7T   316G    90% 327675569  38565763   89%   /Volumes/Timelord

/dev/disk9s2    3.0T   2.3T   663G    78% 570623815 161858851   78%   /Volumes/Historian

As you can see NONE of the disks are full (dev and map aren't really file systems)

When this message appears, the system becomes sluggish.  Explanations please?