Definitely need more info...
Is this a new unit to your setup? Perhaps the drive needs to be set to enable at startup? Jumpers may need setting or if you initialized it you usually have a checkbox to have it recognized at startup when the drivers are installed.
I once had an external drive that had to be turned on a few seconds before booting the main computer and had to have the File Exchange extension active or it wouldn't be recognized as the machine started up. One other thing to note, OS 9 changed the PC formatted SCSI disk mounting to "off" at startup as the default. So if you don't have the proper Apple drivers it may be reading it as a PC disk and not mounting for that reason.
As you can see we need as much detail as possible.