Well, actually there is no actual drive in an iPad, it’s memory soldered right onto the mainboard. Actually I read a lot of comments on the Mixvibes Cross forums about problems with using external storage (sd) on non-Apple tablets. Apparently that is not playing nice with DJ software on Android tablets. Indexes getting messed up, lots of problems for the apps to run correctly.
While I am not a huge Apple fan, the iPad works and it works well. I bought the version with (then) the most memory, just like my current iPhone is a 128GB version. I spend the extra hundred or two for the extra memory, knowing I’d eventually be needing it.
However, if you discipline yourself to using high res MP3 (320) or AAC (256), you are looking at about 100 tracks per GB. Phil Morse (for example) has a total collection of 500-600 tracks, so he could take his entire collection on a 16GB iPad/iPhone easy. With up to 9GB left for iOS and apps.
The point here being to have a compact collection (less is definitely more in this respect) and to use high quality compressed tracks on your iDevice. That way storage space really shouldn’t be an issue. Having multiple iPad DJ apps is fun, but not very productive. Remove all but the software you like best and learn to use it without needing to think about it, prepare your tracks for that software and get playing. You’ll free up loads of space and won’t lose focus 😀
Hope that helps some.