2011 17" Macbook Pro I am trying to re-install Yosemite because Trim enabler is stopping the system booting (tried the fix at https://www.cindori.org/trim-enabler-and-yosemite/ didn't work). It says the last option is to re-install OS X from the recovery partition which will leave programs and data intact but install a new copy of the OS. However when I boot into recovery partition to re-install OSX it says Mavericks not Yosemite. How do I do a re-install of Yosemite ? If I download the Yosemite installer to an external drive can I re-install over the top from there, leaving data and apps intact?