The Apple iPad range is not designed for users to replace or upgrade internal components such as a hard drive. This means you must choose a storage capacity when buying an iPad and cannot change your mind later. You do, however, have several options for working around this lack of expandability.
The iPad 2 is only available to buy new in a 16GB model. The new iPad comes in 16GB, 32GB and 64GB models. The storage is not strictly speaking a hard drive, but rather NAND flash memory. Apple's warranty bars users from opening up their iPad and the physical design makes this extremely difficult to do without damaging the device. This means that in practice you cannot and should not attempt to upgrade the storage capacity or use a third-party service to do so.
One way to cope with the lack of expandability is to control what files you sync to your iPad. iTunes allows you to set different devices to sync different types of files. For example, you may have a large collection of music files synced to an iPod but don't want or need them all on your iPad. To customize syncing, connect your iPad to your computer, click on its device name (such as "My iPad") in the left pane in iTunes, then click on a file type (such as "Music") in the menu at the top of the screen. Here you can select whether to sync all files of this type, sync none or manually select which individual files to sync.
Apple's iCloud service allows you to store several types of files online and access them on your iPad whenever you have Internet access. These files include your photos, music in your iTunes library that is available on the iTunes Match service and any videos you have purchased through iTunes. You can then access and use these files as if they were stored on your iPad.
Several third-party manufacturers produce portable storage devices that can hold files and make them available through a Wi-Fi connection. Check carefully before buying to make sure they are fully compatible with your iPad and that the files will run in the relevant applications.
The information in this article applies to the iPad 2 (released in 2011) and the new iPad (released in 2012) only. Information for other versions and devices may vary slightly or significantly.