stratman70 External USB hard drives are very cheap. I have a bunch. I even one tha's USB 3.0. But an external HD 7200rpm would be better than your 5400. I believe. I always just buy a case and an Int HD and put the drive in. Cheaper and I know what I've got.
John Sonar should stay on you system drive. All your projects should go on one just for projects and if you can a third for samples.
John You have it. Yes.
John It will work just fine. The 3 disk thing is the ideal. It is not a necessity. Many do well with just the one disk. But because you are looking for a bit better performance 2 disks will do the job. I have 5 running disks plus more not running as removable HDs. That is not enough for me.