I bet almost every iPhone/iPad users have seen this annoying message from time to time, unless you are the rich few who always purchase the largest storage option of iDevices. But even then, what's the harm in getting a few more gigabytes of usable memory, right?
Yes, there are a number of ways of retrieving storage space. Deleting unused apps, movies you've seen, backup your camera rolls or even change settings to your chat apps, are what most people do, but it seems like you are giving up something for that extra space.
Is there a way to retrieve lost storage space for nothing?
Of course there is. As it turns out, Apple designed the iPhone or iPad to create hidden cache files during normal operations for time-saving purposes. For obvious reasons, these files can become quite big and caused the above message to pop-up, reminding users that they should probably buy a model with larger storage space next time.
Again, for obvious reasons, there is no way to delete these cache files quickly. But there is a work-around, for the time being:
(1) First of all, launch iTunes Store App and find a movie to rent. Don't worry, you won't actually need to rent it.
Try to find a movie with a bigger size than your current storage space. For example, The Fellowship of the Ring sits at a 6.75GB for 1080p HD.
(2) Tap on the $3.99 RENT button, when it turns green, tap RENT HD MOVIE again. If you've selected a movie that is smaller than your available storage, it will ask you to confirm $3.99 payment and enter your iTunes account password. Tap cancel, and choose another movie with bigger size.
Wait a few seconds, and the screen with "Cannot Download" will pop-up.
(3) Tap on Settings > General > About. Look at the Available storage space, and you will see it's been increased significantly. This is no trick or bug, the retrieved space can actually be used again for any operation.
Of course, this is no magic. The reason this works is because, when you try to rent a movie from iTunes Store, the iPhone will not only calculate the remaining storage but also remove any hidden cache files so that the movie can fit your device. The simple logic is that Apple wants to make you buy a device with higher storage, but it wants you to buy/rent their content even more.
Depending on your previous usage, this method has been proven to magically retrieve up to 2.8GB of storage space. If on your first try, you don't get the full 2.8GB savings, you can repeat this method numerous times until it no longer shows any more storage space saved.
That's it. Give it a try, and let's keep our fingers crossed that Apple doesn't close this loophole any time soon.