I should have added that repairing permissions is a very good maintenance procedure for your mac...
Yes, It's a good idea to either/and run your samples/projects etc from an external or second internal drive (any drive that is not your boot drive running OS X and Logic).
On my imac, despite having a fast internal HD I run my projects from an external, and it makes a difference.
If you have a standard macbookpro configuration it'll have an HD that runs at 5,400 rpm. That's not ideal for audio/video. Most firwire external drives run at 7200rpm which is faster and means your mac can retrieve, find, process etc files more quickly.
There shouldn't be a traffic jam as such because you have a fast bus (no pun intended ;). OSX and Logic will be running from your internal drive and the project files and samples and recordings will be being done on your external.
On my old powerbook i used to run everything off the internal HD but was constantly facing problems under Logic 7. Once I began using a firewire external (even though it was daisychained (linked) to my audio interface I found I could record, playback etc more tracks than before....and that was with a very old Powerbook!
I'm not suggesting that an external drive will solve the overloads issue completely. The first thing I'd do is increase the buffer settings and repair permissions and restart and then go from there.