This symbol has a square in the middle, triangles on the sides, and arrows in the corners. What does it mean?
Since no-one has a good guess, here is how I would try to track it down.


Turn off Time Machine
Reboot the mac
open terminal
run this command sudo fs_usage -w
let fs_usage run for a bit and make sure all disk access is to a minimum,
repeat the steps to make that icon appear - noting the time to the second
press ⌃-C to kill the fs_usage dump and scroll back to see what application or files were being read when the icon appeared.