Can't install SMC firmware update

I'm trying to install the SMC firmware update 1.6 on a mid-2011 Macbook Air. First downloaded it from App Store, and after restart it was stuck at a gray screen with spinning icon, no text or progress indicator-- stayed that way overnight. Tried downloading directly from Apple, same result. Tried resetting SMC, then installing, same result. It won't finish instlaling the update-- any ideas?