With IDE, your limited to IRQ's and number of IDE channels. Each IDE channel uses 1 IRQ,so on a standard motherboard will you use 2 IRQ's and can only have 2 devices per channel. 4 maximum, this is by default. Sure you can add another IDE controller, but you still cannot get up to 15 devices and still use 1 PCI slot! With SCSI, you can have up to 15 devices, all using just 1 IRQ! SCSI is supported by a very wide range of devices, here are just a few: CDROMs, DVDs, CDRs, CDRWs, Hard Drives, Tape Drives, and Scanners.