The ZIP100 Internal is not a DMA capable device so keep it on the secondary channel by itself or with another DMA incapable device (like an older CD-ROM drive). Otherwise the HD or whatever device is on that chain will drop down speeds for compatibility:

Here is how I have it setup on my computer:

Primary Master: DMA capable HD
Primary Slave: 8X4X32 CDRW (also DMA)
Secondary Master: 24X CDROM (Not DMA)
Secondary Slave: ZIP100 (Not DMA)

Works great, everything speedy and I have never burned a coaster with the CDRW. Maybe some mfgr has figured out a way to get around this but this is the way I understand it right now.

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All this technology, and I still can't download a pizza!