TL;DR: WD82PURX and WD82PURZ have similar specifications, but are intended for different regions. WD84PURZ is a lower RPM part number.
WD has introduced so many different part numbers lately that it's hard to keep track. Here is a quick comparison of the different part numbers
|Cameras Supported||Upto 64 HD||Upto 64 HD||Upto 64 HD|
|Max Suistained Transfer rate||245 MB/s||245 MB/s||194 MB/s|
|Cache||256 MB||256 MB||128 MB|
|MTBF||1.5 M Hours||1.5 M Hours||1.0 M Hours|
|Warranty||3 Yrs LTD||3 Yrs LTD||3 Yrs LTD|
|Power Average RW||8.6 W||8.6 W||6.2W|
It is not uncommon to have a surveillance drive with low RPM. Since both power consumption and vibration are low on low RPM drives, they tend to be more reliable, albeit with lower transfer rates.