A couple of things....all leading to an answer of 'don't know, you'll have to try it or hope someone else can verify it'. I know people have used 3TB drives successfully.
Per ASUS, the maximum supported partition size for all router is 4TB for the ARM routers
http://event.asus.com/2009/networks/disksupport/
Most of the problems with the larger drives are due to running out of memory on NTFS format drives. This fork uses the Paragon driver vs the Tuxera driver used on the later levels. Anecdotally, it seems to handle the larger drives better.
Some people have reported that formatting the drives with a larger cluster size can help.
Just an update in terms of USB storage
I do have one RT-AC68P running as media bridge, and one RT-AC68U as main router.
The AC-68P is running a 4 TB WD MyBook (USB3) since approx. 2 years without problems.
I have tested NTFS in the beginning and switched to ext4 later on.
Just one single 4 TB partition.
Until this morning the AC-68U have had a 2 TB disk connected, also no problems for at least 2 years - nonstop connected.
Today my new 8 TB My Book arrived - NTFS pre-formatted.
It works directly on the AC-68U .. the router recognizes the new disk as a single 8 TB NTFS partition.
mount command:
/dev/sda1 on /tmp/mnt/sda1 type ufsd (rw,nodev,relatime,nls=utf8,fmask=0,dmask=0,force)
GPT is not an issue at all !
As I prefer ext4 I did the reformat on antoher Linux machine, parted is missing on the router, fdisk is not feasable to manage over 2 TB.
Now .. the single 8 TB ext4 partition is running smoothly connected to my AC-68U
mount command:
/dev/sda1 on /tmp/mnt/sda1 type ext4 (rw,nodev,relatime,barrier=1,data=ordered)
df command:
/dev/sda1 7752456484 24159804 7337595472 0% /tmp/mnt/sda1
Running on John´s fork -> 22B4
No it´s still not a NAS .. but for me it´s working very well - and stable !