From my experiences with my 2x3T drives i recommend to use GParted aside or any similar livedistro (with parted/gparted tool) to prepare the partitions in advance.
Doing that on shell is not big deal, just few basic commands. On OpenWRT wiki is very nice guide. https://wiki.openwrt.org/doc/howto/storage or https://www.turris.cz/doc/cs/howto/nas
When i was trying to find out what went wrong i was looking for ‘parted’ tool (and during looking why it is not installed by default) i found some article (really i can’t find it again) where was stated that GPT is supported by TurrisOS (like to use it on drives), but due some limitations of correspond fork of OpenWRT it is not recommended to create 2T+ partition(and format them) directly in ssh.