This unit was pulled from a iMac Late 2009 21.5" computer that was upgraded with an SSD. This is an OEM Apple hard drive (see label in picture) that includes onboard temperature monitoring that is used by the iMac to control the internal cooling fans.
The computer was running Mac OS X 10.11.6 (El Capitan). After the hard drive was swapped out, I clean-installed El Capitan on it. When you install it, it will start up at the configuration screen so you can configure the Mac you install it into. If desired, I can put Sierra or High Sierra on it before I ship it to you. Include that instruction after you win the sale. I typically ship my packages within 2 days of sale closing, so please let me know ASAP if you want something different than El Capitan.
Thanks for looking!