After uploading an adafruit sketch related to their GPS shield I found that I no longer could read SD cards on my iMac.
I slipped the card into my MBP and it worked fine.
I then rebooted the iMac and it could now read the card.
Uploaded the sketch again and the computer would not recognise the SD card again.
Another reboot and things were looking fine.
I now reloaded the sketch to my UNO using 1.6.0 and tested and now the computer did remember how to access the SD card.
Something in 1.6.5 is causing this problem.
The tests above show that the problem is related to 1.6.5.
It is serious that the software has a leak into the operating system of the mac, so this needs to be fixed sooner rather than later.

Best regards,