Syncing iPod/iPhone/iPad to a computer may be a piece of cake, but it's totally another thing to sync iPhone/iPod/iPad to multiple computers. At least you will encounter such as a horrible warning message:
The Big Warning Message
This warning message appears because you are using Automatic Mode. Generally, new songs you've added in iTunes would be transferred to the device and the songs you've removed in iTunes would also removed from the device. The computer that this kind of iTunes locates is your iPod/iPhone/iPad's "home computer". Then when you connect your device to a different computer, an alert box will show up as above.
Cancel: Do nothing but keep the music library on your device.
Erase and Sync: Make the new computer its "home" computer. iTunes will delete music library on your device, and then copy the songs and playlists from the new home computer to your device.
Transfer Purchases: If the new computer has been set up with your iTunes Store account and authorized, you can transfer your iTunes Store purchases from your device to the new computer.
Solutions to Sync iPod/iPhone/iPad with Multiple computer
So what's the solution to sync iPod/iPhone/iPad with multiple computers? There are two solutions as below.
Solution 1. Change Automatic Mode to Manual Mode
Different from iTunes Automatic Mode, Manual Mode allows you to keep the songs and playlist on your device, but add extra songs or playlists from the music library on the multiple computers. Choose Cancel in the big warning message and follow the instructions to enable Manual Mode in iTunes.
1. Open iTunes and select your device under Devices on the left-hand side.
2. Click the Summary tab.
3. Check Manually manage music and videos to enable it.
4. Click OK to confirm and click Apply.
5. Done. Now you can drag music and videos from iTunes to the device icon in the sidebar.
Note: iPod shuffle is designed for use with a single computer. You can't transfer music from multiple computers or different iTunes libraries to iPod shuffle.
Solution 2. Sync iPhone/iPod/iPad to multiple computer with a third party tool
Besides iTunes, there is a third party tool, which can achieve your goal. It's named the Wondershare TunesGo Retro (for Windows users). Mac users turn to the Wondershare TunesGo Retro (Mac). Unlike iTunes, this tool empowers you to sync your iPod, iPhone and iPad to multiple computers and iTunes easily. What's important is that it never delete the current music, video and more on the iPod, iPhone and iPad when syncing with multiple computer.
Note: At present, the Windows version has more function than the Mac one. It can sync contacts and manage SMS on your iPad, iPhone or iPod, which the Mac version doesn't support for the time being. Here, I'd like to show you the solution with the Windows version.
Step 1. Install the tool on any computer
Now, install and run this tool on the PC. Connect your iPad, iPod or iPhone data with a USB cable. After that, the primary window comes out, showing data on your iPhone, iPod and iPad on it.
Step 2. Sync iPhone/iPod/iPad with multiple computers
Come to the left column, and you can see Media, Playlist, Contacts, Photos tab. Click each tab to show its management window. Then, click Add or Import/Export to add corresponding files from the any computer to your iPhone, iPod or iPad.
By clicking Playlist > Add > Import iTunes Playlist, you can sync iTunes playlist to your iPhone, iPod or iPad with retaining the current playlists.
If you want to sync another computer, you just install this tool on another computer and do the same procedure. It's quite safe. You don't have to worry ant data loss.