[solved, sort of] 10.11.4 breaks logitech unifying software...
OK, just a glitch - but now somewhat resolved. After the reboot into the 10.11.4 upgrade, the unifying software immediately came up claiming that I had TWO receivers plugged in ( only one was there, in fact ).
Also, the wireless mouse was FROZEN ( though the wireless keyboard still worked). Solved eventually by unplugging the dongle, then plugging it in again. Software still complained one more time, but at least the mouse works now... seems to have fixed itself.
iMac, OS X El Capitan (10.11.4), 32 gigs of RAM, i7, ATI 2GB Graphix