I am assuming your mobo has an IDE port to use.
There are two key things you need to set up: the jumper on the IDE HDD unit for the Master or Slave setting; and, the Boot Priority Sequence.
First of all, STOP thinking that your IDE drive will be a "slave". You have a boot drive already, the SATA unit. Any other HDD installed is just another non-boot drive. Your boot drive is NOT the "Master" of the machine - there is no such thing.
However, any IDE port can support up to two devices sharing the port and a data ribbon cable. To make this work, the devices on the port must be differentiated somehow, and the method is to set a jumper on each IDE device to establish it as either a Master or a Slave. These two terms are relevant ONLY to the two devices sharing one IDE port; they have NO meaning for the machine as a whole. Of the two (possible) devices sharing a port / cable, ONE MUST be the Master of that port, and its jumper set according to the diagram on the HDD unit itself. (Do not use the diagram from another HDD - there is no universal system for all units.) Ideally, the Master device should be plugged into the (Black) connector at the END of the cable. (The Blue connector on the other end goes into the mobo's port.) Even if there is no second device, this is how the first one MUST be set up. (On some HDD's, there are slightly different jumper settings for "Master with no Slave", and Master with Slave Present".) Now, if you also have a second IDE device sharing the same port and cable, it MUST have its jumper set to Slave, and it ought to be plugged into the Grey connector in the middle. If you have a HDD and an optical drive sharing one IDE port on an older machine, sometimes it is best to set the HDD as the port Master and the optical unit as the Slave.
Once that is done, when you boot go immediately into the BIOS Setup screens. FIRST check to see that your IDE HDD is detected properly in the BIOS. If it is not, you can't use it; moreover, if it is generating constant errors, that can prevent the machine from booting at all. Now, assuming it is detected and working, go to the place where you set the Boot Priority Sequence. Make sure that your older SATA HDD unit that you always boot from is the boot device still. Make sure also that the IDE unit is NOT in the boot sequence anywhere. On some machines, by default IF there is an IDE drive installed the mobo tries to use it for booting, and you need to prevent that. Most people would set the Boot Priority Sequence to try the optical drive first, and then the HDD that has the OS on it as second device, and no other choice.