I spent quite a bit of time trying to install the CM11 update, and encountered various errors. When I tried to go back to 4.2 ROMS, I couldn't get them loaded again either. These were fully clean and formatted installs. The errors (and I'm trying to post them as items people like me search on) include Status 7, symlink error, /sd-ext not found, and various other errors such as no error getting reported, but the blue progress thermometer never moving, then saying installation complete.
What I finally found out was when I followed the instructions in the CM11 update thread, it gave a link to the latest CWM 18.104.22.168 recovery, which I used. I finally went back and tried CWM 22.214.171.124 which worked just fine. There's nothing special nor unusual about my particular phone, and like I said, I wound up doing clean installs eventually, so this might be a fairly common problem that might be getting blamed on the ROMs.
So if someone can share this information either back in the CM11 thread or the CWM thread, it might help someone from throwing their phone through the window.