Agree with the response from mende1.
This is the third time my Seagate HD has failed in my 27" iMac. Well, actually two failures, both Seagates, and the trird was after getting Apple's email saying they would replace my HD.
Promptly and efficiently relpaced in my local Apple store and replaced from my TM backup. I go for a belt and braces approach and have three external HD backups. Two TM backups to a TC and a WD disk and a third using SuperDuper to anothe WD disk.
Make sure you format the iMac's Seagate disk, preferrably securely. Like many others you probably have personal and financial data on your iMac which you would not want an errant techie to restore!