hey ryguy76. It so happens that I have a MOTU 828 MK2 myself. Small world. 🙂 I run the 828 on the Firewire 400 port and my two Glyph Firewire drives off of the 800 port. This does force both into 400 mode, but I don't care. I have two other Western Digital firewire drives daisy chained off of the Glyph drives and the rest are USB2. I have my projects on the first Glyph drive, my first samples disk on the second Glyph drive and then more samples on the two WD drives. This collection of drives includes what I think are more intensive sample libraries, EWQL Symphonic Orchestra Gold, EWQL Symphonic Choir, Kirk Hunter Concert Strings II and Diamond, and Spectrasonics OmniSphere and RMX. There are some other smaller libraries, but they don't `count' as they don't need to stream like the previous mentioned libraries. I was careful to keep libraries that require streaming off of the same drive that I would use at the same time. EWQL SO and Choir are on separate drives, Kirk Hunter is on another. Omni and RMX are on different drives. My USB2 drives all handle lighter load libraries. Garritan, some SAM Brass, Chris Hein, Lyrical Distortion, etc. Again, I try to keep libraries I know I will use at the same time on different drives. Currently I have 4 250 GB USB2 drives that connect to 2 separate USB2 hubs that plug into the back of the iMac. I do have other things plugged into the hubs as I have no choice, but it is stuff like iLok, keyboard, etc. whose traffic is minimal. It all works as long as I keep the number of `live' tracks down by freezing or bouncing tracks. It is, admittedly, a pain but I have 4 gig of RAM so I can't complain. The good news is I don't have any crackles or pops because when they do show up I bounce/freeze tracks.
All that and I didn't answer your question! The plug-in itself should go on the OS drive and the sample library the plug-in accesses should be on another. I do have my Logic samples on an external drive, there are no samples that I can think of that are on the internal drive.
I never tried daisy chaining a hard drive off of the MOTU. In theory, you should be able to chain one or two, but I worry about contention (that is part of my `real' job) and thus I avoid any issues by not doing it. I can't say that it won't work well, I have no idea. Hence, it might be worth trying. I would experiment with both high streaming and low streaming libraries to see how they perform. If it does work well post it to the forum as it would be useful to know!
Hope this helps,
-Kevin