iOS 8 update problem... "Fix"...
So, iOS 8 has started rolling out. A lot of pulled out hair. Apparently, if you try to update over the air (i.e. directly on your iPhone/iPad), iOS 8 needs a LOT of space during install. The downloaded software package needs to be unpacked; bits copied here, copied there, and then reassembled.
In other words: you will need more than twice the available space that the downloaded update weighs....
If you do not have enough space, you'll be asked to delete files, photos, tracks etc....
Do yourself a favour:
Either: back up your iDevice to iTunes on your PC or Mac. Do a factory reset, and update to iOS over the air as fresh as can be. Then restore your stuff.
Or: the easiest way: update over USB connected to iTunes on your PC/Mac.
iTunes will prepare the downloaded firmware file BEFORE it is sent to your iDevice.
Thus, you won't need the extra space needed to unpack the update package.