Master/slave were the two "halves" of the old PATA channel, two drives per channel, with only one reading/writing at a time. Standard systems would have two channels, so you could have four PATA drives. With SATA, it's one drive per channel, and usually more channels, like four; so still, four drives, but simpler.
Generally, you could only boot a PATA master; but you can boot any SATA (by default, the lowest channel number). Also, it was a bad idea to put drives that would data going from one to the other on the same PATA channel as master/slave, because only one drive in that channel could be working at a time. But it would still work. Some motherboards also will emulate a PATA controller with two SATA channels/drives -- something you don't need or want, but should still work.
You don't say exactly what the problem is. Does the computer recognize the MyBook drive at all? Does it -- I imagine it's identified as some kind of "WDC" drive -- get listed by the BIOS at startup? If so, try disconnecting the "main" drive, and connecting only the MyBook with the same cables (you know the SATA and power work). With only one drive connected, you should be better able to tell if it is getting power and spinning up.
Or is it that the OS (Windows? Which one?) doesn't recognize it? In that case, boot a Linux CD, and see if it detects the drive.