[Please let me know what is SDD?As you are saying SDD is good for boot drive and WD green is good for data storage so it means there will be two drives one will be used as boot drive and other will be be used as storage drive.Please explain.]
SSD = Solid State Drive, and they're small capacity Sata III drives, and they're offered by several distributors and manufacturers. Go to Newegg, Directron or other large retailers and read about them. Haven’t got one yet, by I’m very curious about them.
As for your question regarding multiple drives, I believe anybody who has been around computers for awhile will tell you the #1 rule is back-up. Rule #2 is always use a second drive as your “working” drive, in other words, only load the basic programs to the OS drive, like MS Office, antivirus, Java, Directx, etc. and load the minor programs you can to the “working” drive. Use the “working” drive to generate, edit and prepare files for storage then cut and paste to the storage drives and let the OS drive do its thing. Rule #3 is always use storage hdds for copy / paste / read / write, only, and NEVER use the OS drive.
Been messing around with banks of laptop drives for storage because they take up less space. Recently bought a bunch of Masscool’s duplex hot swap 3½” front bay enclosures that use laptop drives. With 5¼” housings they can be stacked in multiple bay cases and are easily accessible from the front. Tried it with four of them and it seems to work ok and storage capacity per drive is 1tb with 16mb caches. The tower was networked to the other systems as a faux server, and access to the drives as hot-swap, has been limited. Simply put, once the case was set up, there was no need to swap out the drives unless they crashed, so there sits a stack of 8, 1tb laptop drives in a giant eight bay black box ... Go figure.