I don't know anything about this
I;m not sure because i don't really know, below is what the seagate (the external hard drive's maker) website says.
How do I know if the jumpers are configured correctly?
The correct jumper settings are very important to a successful drive installation. Jumper settings for all Seagate hard drives are available in both graphical and text-only formats. Additionally, all new Seagate hard drives now have the jumper settings printed on the outer casing of the drive.
Most Mac OS systems support multiple SCSI drives and follow standard SCSI configuration conventions. For more information, see our Mac OS FAQ.
Most Macs provide termination power for the SCSI bus, making the most common Term Power setting for Seagate Hard drives "From the Bus." The Mac Plus, Portable, 100-series PowerBooks, 500-series PowerBooks, the PowerBook 1400, and Macs using ATA (IDE) drives do not supply SCSI termination power. For more information on which models do and do not provide SCSI termination, visit Apple's AppleCare Knowledge Base.
Most Mac OS systems that support ATA drives support only one drive, the Master. Make sure the drive is jumpered for Single/Master.
Some newer Mac OS systems support two ATA drives, Master and Slave. Make sure that both drives are jumpered properly for their respective positions on the bus.
If the drive originally came with Mac OS system, please contact Apple (or your Mac OS computer maker) for information about the drive.