Two questions: (1) if there are no other devices attached to the HDD IDE cable, does the HDD still need to be set to "Master"; and (2) I tinker with a variety of computers and their respective HDD's and find that the HDD jumper settings vary from "master" to "CS" to none. What causes this variety of settings? Is it the combination of BIOS and HDD type/size, or what?

I believe that you would have to have the hdd set to either master or cs (cable select) if it is the only drive. I think the choice of cs or master is preference. If you set it to cable select then it does not matter which cable connector it is plugged into, it will auto detect accordingly if it is a master or slave.

Just to add to my previous post, some drives will indicate if there is no jumper then that sets the drive as master.

My belief is that CS was invented by the big box builders so that drives could be added without moving any jumpers on the installed drives. CS requires that the Master drive be connected to the END connector on the cable and the Slave to the intermediate. When using Master/Slave settings there is no such restriction. I recommend using Master/slave. Some older systems won't recognise CS anyway.

WD drives have a separate setting for a single drive on a cable and the master in a two-drive cabling setup. Other than that particular instance a single drive would normally be at 'master', unless you're using cable select.