I'm on a dedicated servers and it's offer max is 1.5TB and I'm already using up 1.4TB, if I go pass this mark what needs to be done? Do I connect another drive and somehow config the 2 to communicate? How is this done?
You need to look at where things are being used up. Is this a Linux system? A new drive can be added, but, you will need to map some of the partitions to the new drive and copy over the info from the original drive in order for it to work. You really do not want to run out of space, as it will have some very strange results on your sites.
1. linux system may require some 10% free space for temporary actions hence make sure you always have space FREE 2. most likely you have nothing to worry huge storage is justified for huge traffic with such huge storage you obviously have 100k or far more unique visitors per day and would rather need to look at page load speed quality with huge traffic resulting from such storage volume it is time to think about sharing your entire load and traffic across multiple dedicated servers