I had (still have them actually, just not using them) two first-generation 1.5 TB Green drives, they both gave me the same sort of trouble after a while... try to do anything with them and they just behave extremely slowly, e.g. try to copy data from or to them the transfer just sits at 100 KB/s for ages then finally after 10-15 minutes picks up at a normal 50-60 MB/s speed. If you're lucky it'll keep it up, but it may drop back down to abysmal speeds. At first I had them in RAID 1 and after learning RAID was bad on Greens, I turned them into independent drives but that didn't help as both drives exhibited the problem. I feel like one drive did it more than the other but both definitely did it.
I contacted WD about it, after some discussion they said I should connect them to a supported SATA controller (I had them on a Promise FastTrak TX4650). I checked with Promise and they said it should be OK but I should update the controller's firmware, which I did, but that didn't help.
I connected them directly to my motherboard since they didn't really need to be on the Promise controller, but that didn't help either. At that point I kind of just gave up yet still put up with the issue for a number of years. I had bought the drives in my home country but had moved here (America) so WD USA would only do one RMA a year for my European drives (idiotic since they come out of the same factory in Thailand) and I just didn't feel like dealing with it or RMA'ing to Europe due to the cost of shipping...
Last year I finally replaced them with Reds.
tl;dr I hate Greens and so do many people but YMMV --- -What is best in life? -To crush your enemies, see them driven before you, and to hear the lamentation of the women.