I have recently upgraded from TS-419P+ (RAID 5 with 3 x 6TB HDD) to TS-464 and added 1 more 6TB HDD. System showed Volume with 4 HDD, instead of getting a usable capacity of 18TB, the system showed only 10.74 TB.
I would appreciate any advise as how can I reclaim the additional 8 TB of usable storage space…
If you moved the drives over, you are stuck with a crippled legacy volume (16TB limit)
You need to start your storage from scratch and restore from backups
Oh… No… Thanks…
So I am stuck… 
Appreciate your response.
You can refer to this FAQ article:
The capacity of my RAID is 16TB. Why do I get an error message when trying to expand the RAID by adding new drives? | QNAP