Not sure if you received any anwers..
Anyway, the use of an external SCSI HDD should be exatcly the same as an internal HDD.
SCSI is a bus technology. The SCSI card (oc chip) is simply another device on the bus. It is a SCSI-to-host bridge (or Host Bus Adapter-HBA). So, if the bus is properly terminated, all the devices on the SCSI bus has the potential to see any other SCSI device, including any OS that have the correct device driver installed.
In general, external HDDs tend to turn off the spining after some idle time.
If you suspect that the HDD is faulty, you can use a SCSI adapter and replce the old SCSI with a new HDD (SCSI 50pin, SCSI 69pin, SCSA).