Is there a special drive for this system?

What every drive you use to replace your current 3.5” HDD will require https://www.ifixit.com/Store/Mac/iMac-21... as the Apple leverages the HDD’s internal thermal sensor to manage the systems thermals via SMC. IF you don’t replace the sensor your system will go into fail safe mode as its dependent on the sensor.

Apple doesn’t use SMART for its thermal management in this series as the version at the time required a IRQ poll which would slow your system. Newer versions of SMART altered this method and Apple altered their systems firmware in the 2014 models to support it with the newer macOS releases.