Do I need a OWC sensor cable

Sadly, you need the sensor to run the system properly if you are swapping out the HDD for the SSD.

Frankly, I find adding the SSD is a better direction! What I’m talking about is Installing iMac Intel 21.5" EMC 2428 Dual Drive Kit this gives you the bang of the SSD but the deep storage of the HDD! Here we also don’t need as massive a drive saving a few bucks. Configuring the SSD as the boot drive with the OS and Apps, and leaving the rest of the drive empty so the OS and Apps have plenty of space for caching, Virtual RAM, indexing and depending on the apps scratch space Then leaving the current HDD for the data drive!

Yes, its a bit more work but in the long haul a better deal.

The reason you are getting conflicting info has to do with the different series of iMac’s! The older 2009/10 had a stuck on sensor which could be transferred to the new drive. The 2011~13 uses the drives internal sensor which is custom to Apple drives. So even a swap out to a new drive gets you messed up! The 2013 21.5” onward models use a 2.5” drive which need to be supporting the newer SMART spec to run correctly. The 2013 27” uses a 3.5” and with the newer firmware don’t need the sensor. So confusing!