Dell Inspiron 7720 slow to boot and open files.

Two days ago I wanted to transfer some files from a flash drive to my Dell Inspiron 7720 and it was very slow but there didn't seem to be other problems. The next day, the laptop started booting very slowly. When I open a folder with files, it takes time to show them. and opening files is slow. Games don't even start (or it probably takes too much time). However, there are no noticeable differences when using the browser.

Is I'm writing this I noticed that files, that are in one of my hard disc drives where Windows and installed programs are, open fine. Files in the other hard disc drive open slowly.

Do you have any ideas what the problem could be?