So moving a server from 15 to 16 that has a storage capacity of 1.5 TB
Used Space 33 GB of 1.5 TB
The upgrade script apparently cannot understand the difference between GB and TB
Error generated
Your system has insufficient disk space i.e 1.5T GB. Upgrade to PBX 16 requires at least 2GB or more of disk space. Kindly free up the disk space and try again.
I haven’t run through that many v16 upgrades but in previous upgrades it will rely on space in /boot. You may need to reallocate some space to that logical volume.