I have a My Book Essential 3TB HD that I want to use to watch movies on my home theater system. I have a Colby LEDTV5536, an Onkyo Ht-R430, and a Directv HD DVR. What are my best options to get this to work?
Well, it would depend on what format the movies are in. Refer to the user manual for this. If it's supported by your system, then you're in luck because all you have to do is establish a connection to the hard disk. If it isn't, you'll have to fork out some cash for a media centre. AC Ryan is a popular choice. If you don't mind a bit of D.I.Y, however, you could consider building a HTPC to suit your needs.
Western Digital Caviar Green 3TB and My Book Essential 3TB Drives Reviewed
Linux is also covered if your distribution enables support for 64-bit LBAs.
Not all external hard drives will work properly when connected to a TV's USB port
DirecTV DVR 2TB or 3TB eSATA Hard Drive Expansion
The best start would be to read your devices manual as to which one supports external storage devices for playback and what formats etc. are supported.
Most likely they will require the drive to be formatted as FAT32 in order to read from it. You may also have to partition it with only a 1TB/2TB partition as support for 3TB is not always given.
Then it's only a matter of connecting it to the selected device and use it's menu to access and playback files from the external storage.