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Why not just plug a USB external drive into your AEBS and use it as your backup for time machine? I don't understand why or what hacks you'd need to do this.
 
If I recall correctly, you can only hook up an AirPort Express to a printer and not either a printer or hard disk. If you have an AirPort Extreme base station that allows you to hook a hard drive up to it, then you can use it with Time Machine. Hope that helps.

~Mullet
 
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