 Recent firmware versions cause the internal disk and any external USB drives to sleep after periods of time as short as 2 minutes.A caveat of the use of AirPort Disk is that the AFP port 548 is reserved for the service, which then does not allow for simultaneous use of port forwarding to provide AFP services to external users.This is also true of a Time Capsule setup for use as a network-based Time Machine Backup location, its main purpose and default configuration.