Q: My system won't boot (running Yosemite 10.10.1 on a 2014 iMac). Is a drive replacement my only solution?

Specs: iMac 27"; 16GB RAM; 3.5GHz quad-core Intel Core i5; 1TB

As of a few hours ago, my system will not boot--except once in safe mode. I've made it to the login screen twice, but it just pinwheels. Diagnostics won't run (I get to the language screen, select my language, and nothing happens). I've repaired the disk, I've verified the permissions, and generally started every alternate way there is to start.

I had this exact issue a month ago. Apple support told me it was probably the way Yosemite installed, so they had me erase and re-install the OS. It worked for a few weeks, until today.

I'd rather not just continually erase and re-install the OS every month, or replace my computer, both of which I am sure will be Apple's answers. So I'm hoping the community has something better in mind.

I didn't know what to diagnose once I logged in via safe mode, but maybe I should start there?