Dish Network DVRs (ex. ViP622/722/922 & Hopper) can transfer (not copy) recorded shows between your DVR and an external USB hard drive. It can playback recorded shows stored on external hard drive but it cannot record directly to external hard drive.
You just need a USB hard drive dock such as BlacX, a typical internal hard drive up to 2 TB and a USB cable. By using a hard drive dock instead of a enclosure, you can swap out external hard drive quickly.
If you want to use 2 TB hard drive, make sure it uses 512 byte sector size and not 4K sector size, otherwise Dish DVRs may not play nice with them.