My iMac fell on a hardwood floor. Power was on at the time and remained on after falling. I quit all applications and...

I'm past the warranty date, so I'm going to order a new HD now. Thanks for the advice.

After receiving your answer, I intended to try partitioning through Disk Utility. However, there seems to be no way to do it. The "partition" tab is not showing. There are only 4 tabs: First Aid, Erase, RAID, and Restore. Is there some other way to partition? If so, how?