It's a common dilemma: you have two computers--perhaps one at work, one at home--but only one iPhone, to which you'd like to sync music and other data from both systems. By default, Apple doesn't allow for this scenario in iTunes. When an iPhone is synced with one computer's iTunes library then connected to another's, all music is grayed out and cannot be synced. Other data are also inaccessible. There are a number of solutions to this problem, some easy but fraught with issues, others more complicated but with more pleasing results.
Trick the library One option is to trick your iPhone into thinking that it's connecting with the same library on both computers.
The easiest way to do this on two Macs is to copy the files
- iTunes Music Library.xml
- "iTunes Library"
from /Users/[your username]/Music on the Mac your iPhone is synced with to the same folder on the secondary Mac (the one on which the music is greyed out).
On two Windows systems, copy the files
- tt>iTunes Music Library.xml
- iTunes Library.itl
from \Documents and Settings\[your username]\My Documents\My Music\ under Windows XP or 2000, or \Users\[your username]\Music\ under Windows Vista on the primary synced machine to the respective (same-named) folders on the secondary systems.
This is a simple fix that works for many users, but it can cause other issues down the road. If you run into any issus after applying this procedure, you can rebuild your iTunes library via the instructions in this article.
Third-party transfer Another option is to use third-party software like TouchCopy. This tool, and others, can transfer music, playlists, album art and more from multiple computers to a single iPhone or iPod touch. It's available for both Mac OS X and Windows--a major boon for dual-platform users.
Deeper modification Finally, if you're willing to dig into text files and modify the iTunes libraries on your two systems, you can follow this procedure from Andrew Grant. The process will modify any iTunes Library so it can also be synced with the iPhone. Per Andrew "You can then either manually manage your iPhone on a second computer, or sync different data on different machines."