I see. What happens when you click to view the Youtube video is your browser begins to download the video file, temporarily storing it on disk. Then your video "viewer" starts loading the file from the disk into its "buffer" (a designated chunk of RAM) and when the buffer is large enough, your viewer begins to play the video. After the video completes, the temporary file is deleted, and the "buffer" is freed for the next video.
The two bars you see are for the downloading process and the playing process.
Happening in Safe Mode pretty much rules out driver issues as Safe Mode uses Windows generic drivers.
Nevertheless, Seatools errors are not good. You can try running Error Checking (or chkdsk /r) from a command prompt) to see if the errors could be repaired, but frankly, I would not trust the drive and would replace it. I am not sure that is your only problem, but since the drive holds the operating system and hardware drivers, I think this should be replaced before assuming anything else.
I will note, however, that heat is also a common cause of sudden freezes and notebooks are notorious for heat issues because of their compact case and the makers' failure
to make essential
cleaning of the interior of heat trapping dust nearly impossible for the "normal" user.
Heat is often responsible for the premature failure of electronics and electro-mechanical devices.