Officially, the iPad cannot be connected to a USB Hard Drive. Instead, your laptop or desktop computer acts as the external drive for your iPad, allowing you to store large amounts of data that you can transfer to the iPad when you need it. Unofficial iPad-compatible hard drives are becoming available as manufacturers learn to work with the new format and the iPad's limitations. However, these devices are not approved by Apple and may affect your iPad's performance.
The iPad has a docking port at its base, used to plug the device into a charger or docking station. This connector also has other uses via Apple's adaptors, such as the USB camera connection kit and the HDMI output connector released along with the iPad 2. The only other port on the iPad is the headphone jack. This limits your use of traditional computer accessories to only those supported by the iPad's hardware and software.
Your Computer as External Hard Drive
The iPad connects with your computer through iTunes, where you can determine which data to sync with the iPad and which to store only on the computer. Photos and videos are the primary storage space hogs on the iPad, and both are easy to selectively sync with your computer. Store all of your video files in iTunes; then, when you want to move a batch to your iPad, select the "Video" tab under your iPad's device listing with the iPad connected to the computer via USB. Choose to sync videos manually, and then select the videos you want to move. Try the same process with the photos, and then sync your iPad to the computer. Your selected items will appear on your iPad, while the rest remain solely on your computer's hard drive. When you want to add more items, repeat the process but de-select the current items and select new photos and videos.
iPad-Ready External Drives
As of March 2011, only one brand of external USB hard drive will interface with the iPad. Apple may withdraw software support of this drive at any time, so purchase with caution. The Sanho HyperDrive works with the iPad USB camera connection kit, available from Apple and its approved retailers. This kit plugs into the iPad docking connector and offers USB and SD card input options. The HyperDrive comes in sizes ranging from 120 G to a full terabyte, but stores your videos, photos and other information in 32 G sections to make it compatible with the iPad's camera connection kit limitations. The drive has a touchscreen interface so you can select which files to share with the iPad through your USB camera kit connection.
The iPad is designed as a mobile device, rather than a primary computer. The developers optimized the device for response time, mobility, versatility and battery life. Supporting a USB hard drive would require additional energy consumption that might significantly shorten the iPad's battery life. The necessary hardware and software to make it as universally adaptable as a computer might also slow the performance and increase the weight. For most users, transferring video, audio and photos between the computer and iPad as needed will provide adequate storage space without an external hard drive.