It appears your system is having trouble detecting correctly the parameters for the Slave unit. Is the BIOS set to detect the size automatically? If it is and it's still having this trouble, look elsewhere in the BIOS Setup screens for a place where you can tell it to detect the HDD sizes for you and let you confirm the proper settings. Usually if you do that you can choose one set of parameters for the HDD manually from a list. In your case, I'm guessing the two HDD's should have almost the same parameters. In fact, if you connect them one way and do this, you should get the correct parameters for the Master unit. Then shut down, switch the drives, and repeat, getting new parameters for the OTHER HDD that is now the Master. Then you should be able to enter these in the BIOS Setup main screen, instead of letting it detect automatically.
Just in case you're having trouble with jumpers, here's a quick review. Use the diagrams on each HDD's case to set its jumper - some are different from others, so don't assume they will be the same. Set one to Master, OR to Master with Slave Present, if that it a different option. Set the other to Slave. When connecting, plug the Master unit into the END of the 80-conductor cable, and the Slave unit to the centre one.