Well, listen me about root on 500GB model i not have the device i have the 16 GB but my work are for both firmware because i produce only vendor and system image without touch kernel and boot image.
I know how the new SuperSu for marshmallow work and i know how the 500GB model differ from the 16GB model.
Maybe the SuperSu not work because can't install in the data partition the su image, the new supersu for marshmallow is completly different from old version.
in marshmallow you need a patched boot image that supersu automatically do, but this is not a problem on 500GB, i think the problem is the small su image in the data partition.
So to solve, if i have in my hands the 500GB i try this:
copy the Supersu 2.68 on a usb disk formatted FAT32
copy the SuperSu 2.49
https://download.chainfire.eu/740/supersu/
start the twrp recovery twrp-multirom-20151112-UNOFFICIAL-foster.img with the command: fastboot boot twrp-multirom-20151112-UNOFFICIAL-foster.img
Install from usb the Supersu 2.68
Install from usb the Supersu 2.49
restart
if it work you have root, if not work at least you must reinstall the firmware but nothing happen.
P.S.
Should work because the first Supersu 2.68 patch the boot image and the second Supersu 2.49 install the classic version in the system.
In Marshmallaw the necessary is a patched boot image with a custom selinux policy, after you can install also the classic root.
Try, i think that could work.
Please try, i'm here for at least other 2 hours for every user that want test. I have also another option if the first not work.
and read the log on the diplsay when you install the first Supersu 2.68, let me know what he say.