Actually, I think this ubiquitous advice to not use internal drive for editing was valid years ago, but is arcane and irrelevant in these days of 80+ MBps drives. I have never ever dropped a single frame due to editing from a system drive. If you don’t use external RAID connected via eSata or fiber or whatever, your simple FW or USB 2.0 external drive will NOT be faster than any recent internally connected one.
I get that the internal drive has a lot going on with OS accessing system files, but you rarely need sustained 60+ MBps throughput, and internal drives can cope with that.