Mxy, isn't SATA boot order determined in the BIOS? There is a concept of primary/secondary SATA channels, but not primary/slave drives. Any of them can be booted off, and the system bootup should try each one until it finds a bootable drive. In my experience, when I plug a single SATA drive in, somewhere along the BIOS/boot screens I see that it recognizes the drive on one of the available channels. It also sounds like the "current" drive is not a SATA drive, correct? If so, and it is a PATA drive, those usually are first to boot before SATA.
Of course you can go into the BIOS, and in one of the screens, you can choose the hard drive boot order, to ensure that your current drive will always boot first.