The MobileSync folder is used by iTunes to backup your iOS devices (iPod, iPhone, IPad, etc.) and, if you are like me and have a couple of these devices, it can grow to a very large size and take up a good amount of disk space on your harddrive. For me, it was taking up about 15GB of disk space. Which doesn't seem like a lot but when you only have a 120GB harddrive that has only 10% free space on it 15GB is a nice savings.

My solution for this problem was to move the MobileSync folder to an external harddrive (the same drive that happens to hold my iTunes library). The problem though is that iTunes doesn't give you a Preference setting with which to move it. So, here is how to move it.

1. Copy or move your ~/Library/Application Support/MobileSync folder to your external harddrive. I typically copy a folder first incase there are errors and then delete the source folder. If an error happens while you are moving a folder you end up with some files in one place and some still in the original and it can be a mess to clean up.
2. Now open terminal and run the following command, replacing <Your External HDD> with the name of your actual external harddrive:
   
   ln -s /Volumes/<Your External HDD>/MobileSync ~/Library/Application\ Support/MobileSync

Essentially what this does it create a symbolic link for the ~/Library/Application\ Support/MobileSync folder that points to the external harrdrive MobileSync folder and from iTunes point of view, nothing has changed and it works as normal.

On a Windows PC, here are some procedures that you can follow: how to move MobileSync backup folder?