I run 5 USB devices and four are external drives.
The drives are NTSF formated to accommodate 60 minute captures in one chunk.
I ran out USB ports and tried a USB hub to add more...no such ruck, you can't run two video drives on one USB hub without a slowdown.
I added another USB card so I'm now loaded with extra USB Ports. I've distributed the drives across 4 different USB connection cards so they don;t bump into each other on the wire or the board.
The software runs on C; I render to another internal drive called "Z" and the four external drives is where I keep raw files. The ProBOB will give me trouble from time to time because it shares a USB card with another drive , but it's seldom used so no big deal.
You can add USB cards until you run out of slots.