To use these special flash drives for iDevices, for the SanDisk devices, SanDisk uses their own proprietary apps to access their drives and access the iPad's operating system to move data back and forth from the their drives from various apps.
SanDisk's apps are available in the iOS App Store for their various iDevice compatible drives.
To see what these drives and apps combinations do seardh the App Store for SanDisk
I use their 128 WifI USB SanDisk connect drive that uses a bulit-in WiFi transmitter to send out a WIFi signal that your iDevice can connect to to wireless transfer , back and forth between iDevices and the wireless drive, various types of data like documents, PDFs, eBooks, videos and images.