I have a T420s and set a BIOS and harddisk password and to boot from the local hard disk only. Additionally I put in a larger SSD from the Intel 500-series.
When I press F1 to enter the BIOS I am automatically in a "read only" mode. How do I unlock this mode?
A user and adminstrator password were possibly set. You need to use the administrator password to modify settings.
In order to enter the supervisor (administrator) password you need to press F1 at boot from a cold start with the machine switched off. When you see the BIOS splash screen press F1.
When the password prompt appears enter the supervisor password which you set. The prompt symbol will be the same as the Power On Password (POP) symbol, but you need to enter the Supervisor password after that the POP password if they are different.