SD card? The Nexus series don't have the facility to use an SD card, only expansion via USB, which in the main is unaffected by this problem.
This issue is the result of Google changing how Android works with those devices that have the ability to expand the internal storage by way of an SD card. It mainly affects those who updated to Android 4.4 from earlier versions.
Its all to do with "file permissions" - Google now only allow apps read & write access to files and folders the app itself has created (for security reasons). They can read data on the card, but cannot make any changes. This means any apps you've "moved to SD card" to save internal memory are unlikely to work.
The developer community are aware of the issue and its hoped that a fix may be issued by way of an OS update, although no one is holding their breath.
Yes, rooting a device generally invalidates any warranty - but for those with older devices that want to use an SD card they way they did before its the only option (as far as I'm aware).
JF