Screwed up my 1TB HD iMac27 - I am running OSX 10.11.5 need to rebuild it in Terminal
Since you have made a full backup, I recommend a full wipe and restore.

120GB plus CoreStorage plus "I had 1TB before" sounds like this is a fusion drive that you have disconnected. 

Reboot the machine in recovery mode (Command-R at power-on) and open Disk Utility. Very easy from there to reformat the internal drives either separately or as a fusion drive again. Then either restore the Time Machine backup or install a new OS and then import your data.

Dealing with core storage volumes in the terminal is very fiddley. Step C depends on the output of step B etc. Quick and easy in Disk Utility.