Hard drive speed won't have any effect on sound quality. It may, however, let you record more things at once or play back more streaming samples at once.
VST samples absolutely can sound as good as any hardware ROMpler. And even though it's considered blasphemy in many quarters, I'll extend that even to software synthesizers versus analog hardware. Computer-based sound sources
can be extraordinarily good, often far superior to conventional hardware synths.
The difference you may be hearing with your hardware samplers is reduced dynamic range compared with computer-based sample libraries. Most hardware synths that play samples (e.g. Motif, Fantom) have a limited number of velocity layers compared to sample libraries on your computer. They are compressed out of the box. That's exactly what you need for live use, but for recording you want to start with wider dynamic range and then squash it to taste. That's what most people mean when they say "radio ready".
As to what makes hardware sound better/different from their software equivalents, that's a
huge topic fraught with controversy and mythology. The short version: you can do what you want with software alone, as long as you learn your tools and techniques really well. Hardware might have a slight edge, but a production done all in software by someone who really knows what they're doing will easily beat a production done with only hardware by someone who doesn't. Expensive hardware is simply not a shortcut to great productions.