Apple's iPad has internal Flash memory and no easy way to connect an external hard drive. But it can be done.
Apple sells a Camera Connection Kit which consists of two plug-in adapters and will cost £25 in the UK from June. They plug in to the iPad's 30-pin dock connector and provide either a USB port or an SD Card reader port. The main idea is to connect either a digital camera or its SD Card and have the Photo app on the iPad read in the stored photos.
What you can do is to connect an external hard drive using the camera connection kit, but the drive must have its own power supply - the iPad won't supply power to it.
Unfortunately, there's no way with the current iPad OS for it to see the hard drive and use it as storage space.
It may be that an iPad app or two will arrive which can use an external hard drive or which will print to a USB-connected printer, if Apple allows such apps to be sold through its monopoly app store. We'll have to wait and see. ®