Can't log in after upgrading to mavericks

Question

After upgrading to Mavericks, I at first seemed to lose the 2nd HD (all data), then list the ability to log in
Help!

Answer 1

If the user account is associated with an Apple ID, and you know the Apple ID password, then maybe the Apple ID can be used to reset your user account password.
Otherwise*, boot into Recovery mode. When the OS X Utilities screen appears, select
Utilities ▹ Terminal
from the menu bar. In the window that opens, type this:
res
Press the tab key. The partial command you typed will automatically be completed to this:
resetpassword
Press return. A Reset Password window opens. Close the Terminal window to get it out of the way.
Select your boot volume ("Macintosh HD," unless you gave it a different name) if not already selected.
Select your username from the menu labeled Select the user account if not already selected.
Follow the prompts to reset the password. It's safest to choose a password that includes only the characters a-z, A-Z, and 0-9.
Select
 ▹ Restart
from the menu bar.
You should now be able to log in with the new password, but your Keychain will be reset (empty.) If you've forgotten the Keychain password (which is ordinarily the same as your login password), there's no way to recover it.
*Note: If you've activated FileVault, this procedure doesn't apply. Follow instead these instructions.