Same Issue
No idea what my friend did with my unrooted and stock Samsung ROM. The device booted and got stuck into Samsung Logo and never went pass that.
On recovery, the Mount /EFS error showed up.
Tried flashing stock ROM via ODIN. (Reset Time and CLear EFS unchecked/checked). All method failed. As of now, for temporary measure, installed TWRP via ODIN and flashed CM13 nightly. Now the device works though IMEI default to 0049XXXX and SIM card not working. Except that all component of the device working. This is very basic and temporary measure.
Don't have the /EFS backup since the device was never rooted.
I'm still finding for a solution.
Samsung Service Centre says that Motherboard needs to be replaced which would cost INR 8,000/-. Thinking of using the diwali sale and exchange this device on the offer. This way EFS and an extra device headache will get over.