Make sure to write down your series links, settings and individual recordings from your to-do list, as once you're on the new drive it'll be blank. You can still shut down, remove the esata, and restart the receiver to go back to the internal drive. Its a hassle but that means you can go back and watch old recorded shows.
The Seagate Showcase and Western Digital DVR Expander both work very well, are exceptionally quiet, and according to my sources will be the two drives that directv supports when they officially support the esata connection. A little more expensive than rolling your own drive/enclosure but improved certainty that it'll continue to work.
It'd be beneficial from a performance perspective to use both the internal and external at the same time, as tivo does when you add a drive to the tivo HD. The internal is used for system stuff/small recordings and the external just for recordings.
The downside is that if either drive fails, you're hosed. Since disk drive failure rates are a fat upside down bell curve with a lot failing quickly or not at all for many years, you've doubled your chances of an early failure. Therefore your failure potential is far higher than half of the MTBF of the drive.
So its appealing to some degree to have both drives in use, in practice you're much better off with a single drive implementation.
What would work great is if they could set it up so the internal drive took on the system files, databases, directv pushed content and other stuff that can be easily regenerated and all the recordings on the external, and allow the external to be moved to a replacement receiver.