Less fans yes, but, the fans in a prebuilt NAS are generally smaller and louder. They also tend to die faster and are harder to replace.
I'd say build your own so you have more expandability and functionality in the future.
FreeNAS, and Unraid are both very popular free operating systems for a NAS environment. Or you can go for something more powerful such as an Windows or Ubuntu based OS.
I would suggest keeping the 60GB SSD in mind, that way your stored files are always on their own drives, independent from your OS.
The only reason I would go for a prebuilt system is if you are worried about the power bill difference. And we're only talking a few dollars a year, nothing major.
edit: No matter which way you go, if you use efficient drives (I suggest something built for a NAS environment), your drives will spin down when not in use. Hell, even my WD blacks in my desktop spin down after a while.
ErrorNumber419 got a reaction from iKatoz in Nas Box or a computer?
