Connecting SCSI devices and installing associated drivers and INITs sometimes cause hardware and software issues.
Hardware issues- Improper termination, typically the cause of hardware conflicts
- Not setting the SCSI priority number to a unique number
- Using a device that improperly "provides" built-in termination, or improperly powers the terminators.
- Damaged cables, terminators, or the terminator power source.
- Low system RAM. Configuring a startup disk with minimal drivers and INITs or adding more RAM may help in a low memory situation.
Software issues
Software conflicts typically result from INITs conflicting at startup. Often, you can isolate a conflicting INIT by removing all extraneous INITs, then systematically adding each one back into the System Folder, one at a time, until you can no longer start up or use a device.
Also, you can often eliminate the conflict by renaming the INIT (or files that contain INITs in their Resource Fork) so that they install in a different order. (INITs and drivers are installed alphabetically.) For example; you could rename the Apple Scanner INIT to "aScanner" to get it to load sooner.
Note: There are some INITs that won't work if renamed. If this is the case, and the INIT conflicts with some device, you may have to remove the INIT from the System Folder when using that device.