C drive.. Primary drive/primary partition (where you installed OS mostly)
D,E,F drives... Secondary Drive/secondary partition (normal drive where you put non-sense

)
If you are buying 2TB no need to worry about this.. when you buy 3-4-5-6 etc and use entire large HDD as a single drive then you will face problem for using more than 2TB space...
because windows and boot, using the GPT files system, where as MBR has a drive size limit of 2.1 or 2.2TB. For more than 2tb do follow these steps.
How to install Windows on 3TB or 4TB and larger HDD - Hard Drives - Storage
So don't worry as of now. Buy 2TB only. I told you this because I thought you will install HDD, and will not partition the drive (C, D, E), So I told you this that primary partition supports upto 2TB..
I think seagate launched 3.5 8TB HDD
Amazon.com: Archive 8 TB 3.5" Internal Hard Drive: Computers & Accessories
So you can use 8TB into 2,4,6 (depending upon your SATA Ports.)

correct me if I am wrong..

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DK_WD
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whitestar_999