AirPort, Back to My Mac: Accessing shared USB volumes using the FAT filesystem on an AirPort Extreme (Early 2009), AirPort Extreme (802.11n), or Time Capsule (any model)
Symptoms
If your AirPort Extreme (Early 2009), AirPort Extreme (802.11n), or Time Capsule (any model) with firmware version 7.4 or later is connected to a USB hard disk drive that uses the FAT (File Allocation Table) filesystem, you may be unable to view or access the connected disk via the Finder sidebar and Back to My Mac.
Alternatively, you may receive a connection alert if the FAT-formatted USB hard disk drive is visible and you attempt to select it in the Finder sidebar.
Products Affected
AirPort Extreme Base Station/802.11n, AirPort Base Station (Dual Ethernet), Time Capsule, AirPort Extreme (Early 2009)
Resolution
You can access a connected FAT16 or FAT32-formatted USB storage device as an AirDisk on your local network, however connecting to such a volume remotely via Back to My Mac is not supported. For more information about supported volume formats, please see this article.