With some exception, most hard drives will work with Apple products. However, getting content from the iPad to the hard drive won't really be like plugging it directly in and drag and drop. Most likely you will need to move content to itunes or icloud and then store onto a mac or pc which will then connect to the hard drive. 
I ordered two portable external hard drives, WD My Passport and Toshiba Arvio (or a similar name).  One for my Windows 10 laptop and one for an iPad 2 and iPad Mini.  I can't remember which is for which system   WD says Windows and WD says Mac.  Is the WD compatible with my iPads?  I know I researched before I bought but before I open these, I need to be sure.  I have amnesia and can't remember things unless always on my mind. Thank you
External HDD