Are you sure you have the 15150N set to a SCSI ID which does not conflict?   I had one (two?) of those way back when.  As I recall they are half height (as thick as an optical drive) and not very fast, but decent enough for a IIci's built-in SCSI bus.

As someone else mentioned, the internal drive is probably set to 0 and the CDROM to 3.   However, I recommend getting a copy of SCSIProbe and without the 15150N connected, use it to identify the SCSI IDs already in use.   Then you can connect the 15150N and work on its configuration.