@95neongirl : I'd try unhooking the 401s, temporarily, just as a test.
PIO and UDMA 4 are totally different animals. I don't know if they'll work against each other, or not. I've read conflicting opinions.
80-wire IDE cable is important, because it adds 40 ground wires between the regular wires - that cuts the static that would mess up high-speed data.
I don't know if PIO is normal for the 401s, the LiteOn site and manuals are kind of, uh, minimalist, on the
technical side. Your computer's bios might be able to change the 401s to UDMA.
The older harddrive probably isn't quick enough to source files to the 1673s.
(But it shouldn't hurt anything if it's just sitting there for storage).
A fast 80-gigger should be enough, for the files you are burning, but you'd want to minimise the multitasking, and keep it defragged. A slow 80-gig might not be. The specs for the 1673s say its max is 21.6 Megs/s at 16x.
The average speed of the 80-gigger should be plenty. But, of course, it doesn't work that way. The platters in a harddrive have fast ends & slow ends, like cds/dvds do. The slow parts of a fast 80-gig would barely be enough. (Most or all 7200 RPM harddrives should be good. I doubt if a 5000 RPM would cut it for 16X burns).
UDMA versions are more about smarter electronics than raw transfer rate.
Harddrives's platter speeds lag well behind their transfer speeds. (Same for SATA, but I digress...).
What kind of motherboard is in there? I've read stuff about NVIDIA controller drivers making a problem, sometimes. VIA chipset drivers have been known to act up, too.