iTunes most annoying flaw ever is that you can’t sync your iOS Device with more than one computer and since most users now have more than PC. This gets really annoying as the iTunes will erase all data as you try to connect your device to another system Here is a solution to that problem.
First you will need: How to Sync your iOS Device With More Than One Computer? Step 1: Head to iTunes folder which is –by default- in
- My Documents > My Music > iTunes [Windows]
- Music Directory [Mac]
Step 2: Copy the files listed below and paste them in another directory an we will be modifying them later to do the multiple sync process.
- “iTunes Music Library.xml”
- “iTunes Music Library” [Tunes Music Library.itl on Windows]
Step 3: Install
UltraEdit.
Step 4: Now we will find the iTunes Library ID against which your iPhone is Sync:
- Open the “iTunes Music Library.xml” with the UltraEdit.
- Go to the entry between the <string></string> tags, after “Library Persistent ID”.
UltraEdit [Windows]
UltraEdit [Mac]
Copy that entry in to a Text file or write it down.
The Next Steps Will Take Place On The Secondary Computer Step 6: Open the File [iTunes Music Library.xml] as in
Step 4 and again copy the entry between the <string></string> tags after “Library Persistent ID” into a Text file or write it down.
Step 7: After you copied that entry, Enter the entry which you copied in
Step 5 on the main computer.
Step 8: Open the Second File “iTunes Music Library” [Tunes Music Library.itl on Windows] With
UltraEdit. Step 9: In “
Search” select “
Replace” for
UltraEdit. 
UltraEdit [Windows]
UltraEdit [Mac]
Enter the Text saved in the Step 5 and Replace it with the text in Step 7 then save the File.
Congratulations, you have done it. Now, Open iTunes on the computer you want to sync with and plug in your iPhone. When you select “Manually manage music and videos” you will no longer be prompted to erase your data and the files on your iPhone won’t be grayed out.