Network volumes, such as NAS drives, Time Capsules, and hard drives connected to network devices are handled differently than drives that are directly attached to your Mac.  As such, utilities such as Disk Utility and TechTool Pro are unable to perform operations on them.  Some network drives have built-in tools that can be accessed through a web interface or third-party software. In the case of Apple’s now-discontinued Time Capsule, you can get some information about the health of its internal disk using Airport Utility.

http://support.apple.com/kb/HT4522