Inconsistent hard disk activity

Perhaps that's not the best title for this question, but I dunno how 
else to phrase it...

I like to rip some CD's with the command:

cdparanoia -Y -X -W --force-cdrom-device /dev/scd0 "1-" - | oggenc -o 
foo.ogg -Q -b 128

I have two CD drives:
  /dev/scd0  IDE0.slave  HP DVDwriter 200i thingie (which is rad)
  /dev/scd1  IDE1.master Sony DDU1621 DVD-ROM

If I use the first drive, I get fast ripping with almost no hard drive 
activity.  However, if I use the second drive, ripping is slightly 
slower and the hard drive is almost constantly going.  Why is this?

CPU usage:
 w/HP drive     40% user    40%system
 w/Sony drive   50% user    45%system

Is it because I'm using a DVD-ROM through the SCSI emulation layer?  ( 
Then why would the HP drive still be fast ).  Is the HP drive more 
"SCSI-emulation-friendly"?  Is it because the Sony drive is just a hair 
slower, so it has to buffer a bunch of stuff to disk?  Is it because the 
Sony drive is on it's own IDE chain ( no shared interrupts or anything 
like that )?  Or is it because there's a rift in the space-time 
continuum in the space between the two drives?

Any thoughts? Thanks.

dannyd

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