Several things to consider.
First off, the boxes are NOT compatible with the new 3TB drives at this time. This may or may not be solved with a software patch in the future, but even in PC's most older computers need a controller card in order to use a 3+TB drive.
Second: Do not cheap out on those "Green" drives - their power saving features tend to cause problems in applications outside of a regular 'desktop' computer. All of the manufacturers make a "Media grade" drive specifically designed for use in DVR's and media servers. Use one of those. I am partial to WD, and if there is a reason that I don't use WD for a particular application, I use Hitachi. I've had bad experiences with Seagates, but YMMV.
Third, look for drives that do NOT use the new Advanced Format. Basically you do not want any drive that says anything about '4k sectors', you want '512k sectors'. The newer ones will work, but what happens is that when the drive controller 'fakes' the old format system it makes the drive do a lot of extra work, and it causes them to fail prematurely.
Fourth - and this is the biggie - the external drive capacity REPLACES the internal drive. It does not add to it. So your unit has (I believe) 750GB internal. You will not get 2.75TB by adding the 2TB external drive, you will only have 2TB. IMO this is stupid and DirecTV should change it, but just don't be surprised when the capacity isn't what you expect. On this same note (which reinforces the stupidity of the setup), you will 'lose' everything stored on the internal drive while the external drive is connected. In order to go back and watch programs that were stored on the internal drive you have to disconnect the external drive and reboot. It's tedious and takes a lot of time, so for all intents and purposes consider the programs on the internal drive gone.
As for setup, All you need to do is buy an external, POWERED enclosure that has an eSATA port. Install the new drive into the enclosure, plug it into the eSATA port on the back of the HR24 (not the USB port, that will not work), and do a red button reboot. The new external drive will be recognized automatically and it will ask you if you want to format it. Once formatted it's ready to go!