The highly regarded Antec MX-1 SATA hard drive enclosure is available at Buy.com for $19.99 after $25 rebate (PDF). Shipping’s free and if you’re new to Google Checkout, you can save another $10. Once you add your drive, simply attach this to a TiVo Series3 for additional storage… TiVo HD owners are going to have to work a little harder. Check FAQ entry #27 for hard drive suggestions.
If you’re in need of more drive space yet find this somewhat overwhelming, you can choose from a variety of pre-assembled eSATA drives (like the Seagate FreeAgent Pro I need to review) for use with a S3 or pick up the TiVo-blessed Western Digital My DVR Expander (currently limited to 500GB) to use with either the S3 or TiVo HD models.
FYI Some of the other DVR manufacturers allow additional storage too (though not FiOS TV as many have pointed out), so you may want to do a little research. Many, like the Dish DVR, use the more commonly available external USB drives — including travel drives without AC adapters. Though, I’m not sure if I’d trust those small 2.5″ guys without cooling to 24hr usage.
(Thanks, Russell)