Hello paalchristian,   For hard drives in the R720XD, you would want to get Enterprise Class hard drives.   I was not able to find specs on W4K81, but I did find 8JRN4 and a couple others compatible to the R720XD   3P3DF HDD, 900GB, 512b, SAS6, 10K RPM, 2.5 inch, 64MB, Seagate, Ironman N9VVV HDD, 900GB, 512b, SAS12, 10K RPM, 2.5 inch, 64MB, Toshiba, AL13SE 8JRN4 HDD, 900GB, 512b, SAS6, 10K RPM, 2.5 Inch, 64MB, Seagate, Compass   Parts & Upgrades also list other options : [URL]   I hope this helps you.  
Hi, I plan to buy some used Dell hard drives for my Dell R720XD 24 bay 2.5.Will any Dell hard drive with correct formfactor for Poweredge Servers be compatible?  while excluding equallogic-compellent drives etc Will for example any of these drives be compatible:1. W4K81 DELL 900GB 6Gbps 10K SFF 2.5" SAS HDD HUC109090CSS600 0B26044 0W4K812. 8JRN4 ST9900805SS
Hard Drive Compatibility Dell R720XD