I recently purchased a 2TB Purple HDD (model WD20PURZ, date: 13 May 2018), and when I connect it to the DVR it would say that it does not detect it ( 12V/2.5A).
I assumed either the DVR is faulty or the drive was not getting enough power.
I read online that it could be that the amperage was low, so I purchased a power-supply 12V/5A
You can refer the link mentioned below and check the power management column to find out the average power requirements under different operations on the drive.