I got 16Gb IPhone. I need to use it as hard disk. Is there any application available to do it?
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You won't find any "Disk Mode" capability built in to iTunes, but there are some third-party utilities that might help....

If you're a Mac user, check out either MegaPhone (http://www.ecamm.com/mac/megaphone/) (formerly iPhoneDrive) or iPhoneDisk (http://code.google.com/p/iphonedisk/).

If you're on Windows, TouchCopy (http://www.wideanglesoftware.com/touchcopy/) is another option (the iPod touch and iPhone use the same basic operating system).
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