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Unless there is something screwy about the My Book, then the answer is yes. I have 2 USB disks (generic cases with scavenged desktop drives) simultaneously connected to my AEBS via a hub.
 
I think I remember this not working when the Airport Extreme was first released, anybody know what I'm talking about or have I just been dreaming?
 
I think I remember this not working when the Airport Extreme was first released, anybody know what I'm talking about or have I just been dreaming?

The product documentation suggests it will work, and my AEBS is the older version with the 10/100 switch, running the current firmware release.
 
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