Some keyboard keys not working on password input to Yosemite login screen
Solution 1:
The fact that the keyboard works at the boot screen before user level code runs is curious.
I wonder if one of the kernel extensions or control panels that customize that keyboard are causing issues. As I don't have that hardware, it's hard to poke and see what files might be causing the issue.
You can try several things to corner this:
- Make a new user account and turn off auto log in in the Users preference pane
- You could hold shift down when booting to rebuild all kernel extensions and then log in safely without most third party apps running
- You could uninstall any drivers and/or open a support ticket with the vendor
In each of the above cases, you can test logging in to the new account / safe boot system and see which are broken and which are working. Worst case, edit the question and/or open a new question with the details of this triage.