Tried unsuccessfully, two times, to update my iPad Air to iOS8.2b5. Both from the iPad itself and from Option+Clicking the file from iTunes 12.1.
Trying to do a restore back to 8.1.3 and then update to 8.2b5 to see if it works.
**Just reporting in case others are in the same boat
Now a corrupt download of 8.1.3 from iTunes.
Downloaded a fresh install of 8.1.3 direct from Apple outside of iTunes.
Restoring 8.1.3, seems to be continuing through the restore process just fine.
It did.
Jumping straight into update of 8.2b5, failed again. Unknown Error (14)
Trying updating to 8.2b5 iPad 4th gen, seems to be working fine.
Re-downloading 8.2b5 from dev page. for iPad Air1
Issue appears to have been a corrupt download. After re-downloading latest build from dev page, the iPad Air1 updated without issue within a couple minutes.