We are hearing reports of a bug in the Airport software version 7.4.2 -- access to hard disks attached to the AirPort via USB become unusable after a few hours. You can't even access them through the Finder until the AirPort reboots. It's discussed in this thread on the Apple Forums, but there's been no official comment from Apple.
The solution is to revert to software version 7.4.1. To revert, run the AirPort Utility, click on the Summary tab (once you're in Manual Setup mode), and then click on the word Version -- this brings up a list of previous software versions, making it easy to move back one version.
[robg adds: I was having this problem myself, which was really bad as my Time Machine backups are on the attached disk. Every few hours, I'd have to reboot the AirPort to get Time Machine working again. Downgrading to 7.4.1 has solved the problem (going on two weeks now without a single failure). Note you can also reach the firmware options screen by selecting Base Station -> Upload Firmware.]
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