Apple released AirPort Extreme and AirPort Time Capsule base stations Firmware 7.7.2. The maintenance release addresses a storage issue, a WAN connectivity issue, and another networking issues.
Apple's patch notes:
Firmware update 7.7.2 is for AirPort Extreme and AirPort Time Capsule base stations with 802.11ac.
- Addresses an issue that prevents a USB hard drive connected to an AirPort Extreme from appearing as an available disk in Time Machine
- Resolves an issue that may intermittently cause a loss in WAN connectivity
- Resolves an issue that may result in a network loop
If your base station or Time Capsule is eligible for the update, you'll find it in AirPort Utility. Apple has a KnowledgeBase article with additional information about AirPort firmware updates.