How to set up new iPhone without deleting its data?

I had my first iPhone synched with my Macbook, then I bought a second iPhone, a 3GS, and I still haven't synchronized it with my Macbook. In fact, I'm not sure I set this 3GS up with any computer at all: I don't remember setting up this new 3GS with any computer (is there any way I can doublecheck this from the iPhone itself?).

I have been using this 3GS for months, and now it's full of vital data that has not been backed up anywhere. Also, its iOS has never been updated. For these reasons, I'd like to synch it with my iTunes, so that I can back it up, and update iOS.

When I connect the 3GS to my MacBook, iTunes 10 warns me that:

"This computer has been previously synced with and iPhone or another iOS device"

I then have two options: 1) "set up as new iPhone" 2) "restore from the backup of" (here I see my first iPhone's backups)

Obviously I don't want to overwrite my current 3GS data with backups from another phone. Also, I don't want to erase or loose any data currently present on my 3GS.

My question is: how can I backup the data from this 3GS without loosing anything ?

I strongly fear that choosing "set up as new iPhone" will erase everything currently present on my 3GS.

Firstly you have to configure iTunes not to auto-sync.

Then, you plug your iPhone, and back it up.

The real problem here is coming to grips with how iTunes backs up data and what it means when the sandboxed data gets restored onto a device that may or may not have the same apps. Second - you have two distinct problems. First - making a backup and knowing it will be restorable if needed. Second - breaking the sync pairing between the old iTunes library and setting up a new pairing with your current computer (if desired)

Here are some articles that cover the basics:

• iOS: How to back up
• iOS: How to back up your data and set up as a new device
• MobileMe: Backing up your Contacts, Calendar, and Bookmarks if they only exist on an iOS device
• iCloud: How to back up iCloud data
• iOS: How to transfer or sync content to your computer
• Using iPhone, iPad, or iPod with multiple computers

Over time, Apple keeps making this better and better, but it really depends on which version of iOS and which version of iTunes you are using. Also - some data will never be backed up (account passwords when data protection / backup encryption is not enabled).

Three strategies exist:

1) Just do the work to sync only the data you need and avoid backing up - go over each app on the phone - delete the ones you don't care to back up data (games, unused apps, trivial ones to start again like Facebook or Twitter where the app only caches data - not stores the only copy of data)

2) Let iTunes back up the phone and test restoring it on a new device. You might be happy with iTunes and can live with things not needing to set up a new device.

3) Send the backup to iCloud and see about restoring after you've gotten the device paired with the new computer.

The big problem is you don't know if the backup is good until you test restoring it and most people don't want to go to the hassle to test things to make sure the don't "loose anything". Also - each person makes different choices on how and what they care to sync (and by which method) so a general list of steps becomes like a "choose your own adventure" book with dozens of decision points where the steps diverge.