Basically the title says it all, but let me elaborate. Two nights ago just before switching it off and going to bed, i noticed my MBP was acting a little weird. I would get the rainbow wheel every now and then. It didn't bother me as i thought that it would be working fine again on the next boot. When i woke up the next morning, it was worse. It was practically impossible to do anything without the rainbow wheel showing up. I thought it might be a RAM or CPU issue. So i managed to open Boost Up Memory and freed some memory. The CPU and RAM was ok but i was still getting the same issue. I couldn't reboot my MBP so i shut it down abruptly and restarted it again and that's when i got the "Question Mark folder". Just for info, my MBP is equipped with a 1TB HDD, 8GB RAM and El Capitan. I found out that it's a common issue with the hard drive cable on some MBP of the same gen. But mine was working FINE until two days ago. More over, i also found that some users have fixed the same issue by rebooting in safe mode or verbose mode without actually changing any hardware. For now, luckily i still have my intial HDD with the everything on it and just swapped it. It's working ok, but still some minor issues like Finder crashes every time and won't open. I'm considering booting my MBP on the 1TB via USB and see if it works or not. Then download and install macOS Sierra and swapped it again to see if the problem still persists. If not, i'll definitely have to change the HDD cable. Has anyone encountered this same issue and managed to fix it on a software level?