Drive designation

I have noticed that on a scsi system (one HDD + IDE DVD) with a USB ZIP drive connected, the HDD is set to sdb rather than sda! The ZIP was set to sba4 - no other sba's seen. I was surprised that the device was seen as a scsi device (sd?) and that it had priority over the fixed disks. I haven't used USB on LINUX before. Apart from obviously disconnecting the ZIP, is it possible to override the drive designations during installation? Adrian