Change the Location of Your iPhone Backup

Are those iOS backups taking up too much space on your boot drive? While there’s no built-in function in the iTunes preferences menu for moving the archive folder, it’s a simple procedure on most platforms. While others have mentioned this in the past, I haven’t come across a single page with instructions for all major operating systems. For reference, the instructions are below:

Mac OS X
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■Close iTunes
■Move the existing ~/Library/Application Support/MobileSync/Backup/ folder to the destination drive (for example, named BigExternalDrive)
■Open a command prompt by launching Terminal and create a symbolic link using a command similar to the one below, replacing /Volumes/BigExternalDrive/Backup with the path to your destination:

ln -s /Volumes/BigExternalDrive/Backup/ ~/Library/Application\ Support/MobileSync/Backup/

Windows Vista and Windows 7
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■Close iTunes
■Move the existing C:\Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\ folder to the destination drive (for example, D:\)
■Open a command prompt and create an NTFS junction point using a command similar to the one below, replacing D:\Backup with the path to your destination:

mklink /J "C:\Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup" "D:\Backup"

Windows XP
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■Close iTunes
■Download and extract this junction utility to your Desktop
■Move the existing C:\Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\ folder to the destination drive (for example, D:\)
■Open a command prompt an NTFS junction point using a command similar to the one below, replacing D:\Backup with the path to your destination:

1. cd Desktop
2. junction "C:\Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup" "D:\Backup"

Source: http://oliveraaltonen.com/2011/01/03/change-the-location-of-your-iphone-backup/