You can use your iPhone big memory (8GB, 16GB, 32GB) as a flash drive.
There is a popular app for turning the iPhone into the flash storage device is PhoneView, it is previously known as iPhone Drive and MegaPhone. There is a free demo available for this app, but it is only for Mac usres. It costs about $20. If you are a Windows user, there is an app TouchCopy and it costs $25.
Here is the source link for download