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.
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