Q: Upgrading OSX after migration

I've upgraded my SSD to larger size by installing the old SSD into a an external case and migrating it to the new one which I installed din the MacBook Pro. I want to upgrade from Lion to El Capitan but it won't allow me because it's recognising the new disk as a Time Machine back up. The new SSD has been used for Time Machine in the past but I wiped it before installing it in the MBP. I've tried going into TM and deselecting the disk but it won't allow me to do so. Any idea how to do this