The biggest drives today are 2.1TB that is all that Windows can recognize at the moment but that is all about to change soon a couple manufactures are working on getting bigger drives working. Seagate is the closest right now on a 3 TB drive and I believe Western Digital is working on a 5 TB Drive. Now in server applications where they have huge arrays of multiple drives the amount of storage space is only limited to the size of the room they are building the server in.
An exabyte (derived from the SI prefix exa-) is a unit of information or computer storage equal to one quintillion bytes (short scale). It is commonly abbreviated EB. When used with byte multiples, the unit indicates a power of 1000:
1 EB = 1,000,000,000,000,000,000 B = 1018 bytes = 1 billion gigabytes = 1 million terabytes
The term exbibyte, using a binary prefix, is used for powers of 1024 bytes.
In principle, the 64-bit microprocessors found in many computers can address 16 exbibytes, or just over 18 exabytes, of memory