Hmm, that's strange, given that Sierra, OS 1012.6, is a very stable OS. When did this start happening? Have you done any software installations at all on your iMac?
Also, the typical questions:
1. Have you ever done an disk cleanup/maintenance/repairs, from a software perspective? By that I mean have you ever run any disk-related software to help with those tasks? There are some excellent products available (both free and commercial) that can help you with that.
2. This is critical: are you making backups to an external device? If yo are, what software are you using for that?
3. What third party software are you using?