I phones or devices synch with one computer. If you change computers it will ask you to erase and sync.
To get your content of the iPhone this article gives the answer
Ok, That´s why I don´t use iTunes. It always says that i´ts gonna delete all my music and other stuff on my phone. Thats´not sync to me. Synchronizing to me is to insert information that is missing (or is new data) on anyone of the units. Not deleting. I can´t see that, in a few years time, anyone would want to use iTunes if it keeps work like it does today. Really gets med mad each time i try to understand it. If Apple wants to make it difficult to use other media than the media bought in iTunes they succeded.
iphones are not storage devices and are not to be used to transfer music from one computer to another. Older Ipods can be used as hard drives.
If you want to copy to another computer here are your options
If you have any feature requests please use this link
I have two kids, one wife, one house, two cars and i work 7-17. I have no time to go to apple University to learn how to sync my data. It should just work. drag and drop. copy-paste. And my originally synced PC doen´t exist anymore. That diks crasched. And I always experiment and try new installations of diffreent OS:es. So why should I stick to just one single computer in the world if i want to sync. Ridiculous. Result: I don´t sync. At all.