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I already have the 2Tb drive so want to use it. There is only room for one extra drive in the PC case.
I got it "free" when I bought a NAS drive a year or so back (special offer) but the NAS drive had only just started supporting 2Tb drives so I put a 1Tb drive in it instead just to be safe.
I already have about 750Gb of data backed up (years of computer work) so need to look at hard disks larger than 1Tb.
Note Western Digital now have a 3Tb drive available.