MacBook Pro 2012 15" can’t boot When I turn it on, it shows the Apple logo and starts loading up to about 50%, then the screen turns black. Running the hardware test shows this: 

Alert! Apple Hardware test has detected an error. 

4MEM/61/40000000: 0x79d53990




Running the system file diagnostics shows this:

 

What error is that? How can it be fixed? I don’t have a back up and can't risk losing my data.
The error code indicates that the Apple Hardware Test tested your RAM-modules and found a problem with at least one memory module.  In order to solve this you'll need to have the RAM module replaced (i.e. hardware repair).

If you have the 15" MacBook Pro (mid-2012, non-retina), you can buy a new RAM module yourself and replace it fairly easily.

If you have the 15" Retina MacBook Pro (mid-2012) the RAM module is soldered on the logic board and not easily replaceable. In this case, contact Apple or a repair shop for help.