Hey, smalltech!
To answer your question directly: Yes, the WD Green is suitable for your needs as it is designed exactly for that same purpose - to be used as secondary/backup storage internally or externally (put in enclosure). They offer large capacity for your massive storage and run cool and quiet. They do have the power-efficiency feature, as it was already mentioned, which makes them spin down when they are not accessed but they are still fully functional once you start to read/write data. Here's a link to the product page of the WD Green drive, where you can see all the features that it incorporates:
http://products.wdc.com/support/kb.ashx?id=RYr6no
Let me know if you have any other questions!

SuperSoph_WD