Well if you have a Mac you can import your photos into iPhoto. When you do that it will allow you to delete the photos off the iPad to free up space.
If you have a PC..well I can't say for sure. Open iTunes on your PC and plug in your iPad. Then go to the photos tab and it should ask you were to sync your photos on the PC.
Edit: If it's a storage device like a USB or external HDD you were referring to, your still gonna need to do the above process. As far as I know you cannot transfer directly from iPad to external device(unless rooted maybe?) From your computer you can further transfer them to an external device.
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