Used dd to copy and half of my new disks storage capacity is unavailable

Your filesystem is reporting 2TB of available space, not the disk.

Your old disk had a 2TB partition defined, so after copying the entire thing (including the list of partitions!), the new disk also has a 2TB partition defined¹. If you take a look using gparted or gnome-disks, you'll probably see a 4TB disk with half of it in use by the data partition and the other half 'unallocated'.

Use GParted or some other partitioning tool to expand the partition to the entire disk.

(Once the partition is resized, the ext4 filesystem also needs to be grown to fill the new capacity, but GParted will do that automatically. With some other tools you may need to run resize2fs manually.)

¹ (Note that as far as the disk knows, partitions are just data – they're only recognized by the OS, not by the disk itself, and having smaller partitions cannot cause the physical disk to report a different capacity.

Partition definitions i.e. start–end ranges are stored in sectors 0–33 of /dev/sdb, and any tool that copies the raw contents of /dev/sdb will end up transferring the partition table too.)