If you're running Windows, Task Manager can show you disk usage in the "processes" tab. One of the programs is probably constantly running the disk. It might be bugged or it might be malware, etc.
Or, your computer doesn't have enough RAM and it's having to use virtual memory, which really slows a computer down. Again, it will show your RAM usage in Task Manager.
If it's Windows, it could be indexing the hard drive, that is making a table of all the files on it so searching for something goes faster. I usually turn this off personally because I don't spend that much time searching for files.