Hello, I have a setup in my house where I use three Airport Extreme's (two of them extend the range of the first one). They are 6th Gen, 5th Gen, and 4th Gen respectively. The 6th Gen is plugged into my Cable Internet. I had a WD 4TB HDD plugged into the USB port of my 5th Gen. And previously it was plugged into my 6th Gen. This is a very convenient setup for me because I have numerous laptops that I want to backup. Every once in a while the time machine backup fails and the "EFI partition" as well as the "Timemachine Backup" part of my newtworked HDD loads onto my desktop. Once this happens the HDD will not start backing up again on its own no matter what I do. If I plug the HDD into my laptop and run disk utitilty it usually finds some errors which can be fixed. Once these errors are fixed the timemachine backup process starts working again... until the next time it happens... I have sent three WD HDD's back to WD under warranty and received new ones each time. But still after a period this same exact problem keeps happening. Do I keep getting bad HDD's from WD? Or is there nothing wrong with my HDD and some other problem is going on? Please help I am very confused and looking for an answer to my time machine backup problems. Oh I am currently running OS 10.9 and all my Airport basestations have the latest firmware but this previously also happened under the older versions of OS X too. Thanks!