I was working in Logic Pro today and all of a sudden I got the spinning beach ball which is extremely rare. I closed Logic, tried to relaunch but only got the jumping Logic icon. Next I tried a reboot (of El Cap, latest update) from my SM512 blade. This usually takes 15-20 seconds. But right after completing the progress bar the Mac just shows a white screen and hangs indefinitely. I changed the boot drive back to the built in spinning 1 GB WD which still has an outdated version of Mavericks, with exactly the same result. The Mac just hangs at a white screen. Same when attempting a boot to save mode. Resetting the NVRAM seemed to work (judging from the sound) but again only brings the Mac to a white screen. Diagnostics doesn't seem to diagnose anything, the Mac just hangs at a white screen with the mouse pointer shown but no reaction to movements on my magic mouse. Entering recovery mode works incl. magic mouse and internet access, and I'm currently installing El Cap over the internet onto the original boot drive. Is there anything else I can try/do? It's the Mac Pro in my sig that has been working like a charm since '12 when bought refurbished from Apple. Incredibly solid/stable system until now I'm pretty frustrated, this is my production Mac Pro, there is no "junk" on this machine, only Logic Pro and tons of sample libraries.