This is an interesting question from a blockchain perspective. Will the transaction block data ever outstrip the ability to store it ? Or at least put it beyond all but the largest corporation's budget? It seems like storage capacities are still growing at a very fast pace, so I don't think this is an immediate problem.
But from a device standpoint, there are many schemes to create meta-volumes to the which physical devices may be added without affecting the ongoing operation of the system. Linux's Logical Volume Manager (LVM) is just one of these.
There are also any number of disk-array schemes and commercial network attached storage (NAS) units. The Drobo name is just one that comes to mind.
To specifically answer your question: if the external 1TB storage unit you are using is approaching its capacity, it would be time to upgrade. Acquire a larger unit, or move to a different system (perhaps one with the ability to add hardware storage on-the-fly, such as server able to operate an array of multiple storage units.)
Briefly stop your coinservers, attach the new, larger storage, copy the original blockchain data to the new storage unit, reconfigure the coinserver to operate from the new unit, and voilà.
You would add a line similar to this in your ~/.bitcoin/bitcoin.conf file:
Multiple disks can be arrayed in different ways depending on the competing priorities of redundancy vs. data access speed. SSD's are plenty fast, and even rotational hard disks with enough cache memory are sufficiently fast. I would suggest a mirrored array (RAID-1) of rotational HD's as the best option.