If it's single, toss the jumper. If it's master over some other device (such as the CD-ROM), set it as master. If it's slave to some other device, set it as slave.
WD uses a jumper for Master, but no jumper for Single. Single means there's nothing else on that IDE cable.
WD actually puts the jumper across a couple ground terminals for "storage" when they ship the drive.
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