I am having this problem on my 2012 MBP Retina. I have a Time Capsule running 7.6.4 and the only versions available for me to roll back to are 7.6.3 and 7.6.1.
My MacBook Pro will be asleep (Power Nap is enalbed) and my external USB drive plugged up to the Time Capsule will spin up, then immediatly spin down. Over and over again, back to back. It seems to stop when I wake up my MacBook Pro and start using it.
I do have the drive mounted on the desktop before I close the laptop. I would sure like to know what causes this, though, as it's probably killing the life of my external hard drive.
That is fine.. but I would like to also know if the problem is related to how powernap works.
What OS is the MBP running?
Can you put drives to sleep with specific setting. Even if you set it to something other than default just to try what happens.
In the end.. the problem is Apple do not provide end users with low level access to the router. The later the airport utility the more stuff they removed.. you do not even have a log. So when these issues crop up it is often users who solve it with info like which firmware has the problem.
I can get you a bit deeper into the system but it won't be easy. You may need to do TCPdumps via managed switch or hub which is pretty much in the field of the expert rather than garden user.. and even if you find the problem there is no reason for apple to provide a solution.
I'll turn Power Nap off and see if I ever notice it happen over a few weeks.
The MBP is running the latest 10.9.2. I have no way of specifying sleep intervals for the hdd. The option to "put hard disks to sleep when possible" is checked in Energy Saver, but I'm not sure if that even applies to NAS. It might be something in the hdd firmware that controls that sort of thing. Either way I can toggle that when testing, as well.
Yeah, Apple does appear to be "simplifying" the capabilities of the AirPort Utility application, while stripping out power user features. I can't even open AirPort Utility 5.6 anymore under Mavericks! :-(
While I'm certainly comfortable performing advanced troubleshooting, I'd hate to waste your time when it might not get us anywhere in the end.