Usually a failing HDD, can be bad cables, heat or a motherboard issue. Advice? Change the cheapest part first.
So, I have no idea where to start,
2 days back everything was working just fine. I wanted to have linux and win 10 on the same disk so I downloaded linux, did bootable usb, installed it to the same disk as win 10. After restart linux won't boot and win 10 isn't in grub. I tried fixing that, but failed, so I simply reinstalled win 10. Everything seemed fine, but after while my second disk just disconnected. After restart everything was fine, so I started downloading stuff and it just disappeard.
I have no idea what I was doing and what I should do know.
Sorry for my bad english.

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