recently on 1 of my NAS boxes a Synology ds411j i was replaciong my 4TB drives with 6TB ones and trying to expand the size I should have around 20TB available right, but no there is a problem going past 16TB it seems. In netapp land this was a 32bit aggregate problem but ontap can convert aggregates tro 64 on the fly once you add disks to go past 16TB.
The heart of the problem is the 32bit cpu actually, in 1 register you can hold 2^32 *4096 bytes for the page cache(buffered writes) thus you can not exceed 16TB until you go to a 64 bit processor is the bottom line.
when i looked into my synologys cpu it is this, which made me laugh:
MARVELL Kirkwood 88F6281
Marvell???? I thought they were a comic book company.
som screenshots of the fun: