will try to answer, but its my own opinion and will be incorrect
the tabs 16g is split up into sections. we only get to use 13ish
1stly, lets go back to GB partitions
we have system - where rom goes
app data - applications go here
cache - dalvik cache
user data - our internal sd
as android developed, things changed.
cache files became stored in app space. apps could go in user. we could move stuff from app to sd.
clever people used this cache space for other things. (see supercharger for example) eg swap space
(at one point the omni nightly had a huge app partiton)
now lollipop has done away with dalvik (moved to art) the cache partition is totally wasted
i would want this space back...
our great devs have done so and now we have
system
app data
user data
great, lollipop woo hoo
ahh... now you want to go back...
well all the previous roms expect one more partition...
i would guess the missing sd card is the last partition so expect the previous one (guessing cache) is 12G....
and cache is unused...
not good hey
using odin and ticking the repartition box will relay the partitions based on the boundary sizes specified in the pit file.
of course this means trashing ALL data on the tab.
which means we lose all partitions (recovery, modem)
the safestock files / etc will put a version back on.
then a custom recovery
then flash away....
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been here for a while and dont need to post too often
the tab forum is better than others i also look on.
just a reminder, this is a nightly. i expect things to break,
and there's motivation on finding answers when you have bricked your device