My work history and background prior to filmmaking was in audio engineering and production. Since I have ventured into filmmaking years back, I have built a sound library that just exceeds 1.5TB.
Building and maintaining your own sound library is definitely an essential element in being able to find and access the right audio elements for any production. There are many companies out there that make sound effect, foley, and sound design kits specifically for film productions, but much like any other store bought libraries, they can often be hit or miss. The "Hollywood Edge" libraries are pretty decent, and contain a wide variety of sounds specifically for TV, Radio, and Film production.
The one thing I can say is that you should always be searching and adding yo your sound library. If you can, go out and buy an external drive specifically for your sound library, and develop a filing and organization system that works best for you. Much like the previous post, I have main folders dedicated to: Sound Effects, Foley Sounds, Sound Design, and Music Beds. From there, you can create a sub-filing system as specific as you'd prefer. I have mine pretty well organized into probably 200 subfolders. Ive also created a database using Filemaker to easily organize my contents.
I also agree with the previous post about SoundMine and Snapper. They are both pretty awesome applications for previewing and importing sections of an audio file directly into your DAW, though I do concur the Soundmine is a more versatile application.