I like the 4T Seagate "portable" (i.e. 2.5") external 4T disk drives. They are good for backups. - 2.5" drives are physically small - 2.5" external drives only have one (USB3) cable. No separate power cable and wall-wart (3.5" externals require these). - If you don't have USB 3 on your computer I'm not sure that the drive will get enough power to run. Even if it works, filling a 4T drive at USB 2 "Full Speed" is going to take a very long time. - shuckable (i.e. you can pry off the case and use the raw drive). This is good if something goes wrong. It isn't true of Western Digital drives. - unfortunate: if I remember correctly, the physical raw drive is a little thicker than most 2.5" drive bays allow. So shucking to get a raw drive to install inside a notebook may not work. The Seagate 2T portable externals do work. - 2.5" drives seem to handle mechanical shock better than 3.5" drives - 2.5" drives are a lot easier to store than 3.5" drives (a lot smaller, lighter, and fewer bits) - cheap. Usually $125 is possible. Last I looked, cheaper than the internal drive. Right now Staples is selling these for $99.92 + a little environmental stewardship fee. And there are a lot of $15 off $100 Staples coupons floating about. I even had (and used) a $25 off $75 coupon. <http://forums.redflagdeals.com/staples-staples-seagate-expansion-stea4000400-4-tb-usb-3-0-99-92-additional-savings-coupon-2218659/> (For most internal applications these days, SSDs make more sense than HDDs. SSD prices seem to be starting to fall. For a number of our notebooks we actually have both.) There are drives with larger capacity but they are generally more expensive per byte, shingled, or 3.5". I don't yet trust shingled drives but a natural application for them would be backing up because that doesn't require random access writing, their weak point.