Hmm, well i'd assume you are running from IDE1 on your motherboard for the hard drives. IDE2 is usually used for all the cd/dvd drives, etc.
Now, there should be one cable coming off the IDE1 port and to the Slave drive connected using a connector in the middle of the 1 cable. On the other end of the cable is another connector which should have the Master drive connected to it, so they should both be linked together.
Try this, and I hope it works, though you might have it set up like this already:
Is your slave drive blank? If so and you have Windows XP, right click My Computer icon, and go to Manage.
Select disk management from the list on the left.
Now all the hard drives should be on Disk 0.
Right click the Slave drive and select format from the list.
Follow the instructions, selecting NTFS from the list (This file system only works on the latest windows and performs and works a bit differently, buit its better than the old FAT 32).
If you have anything less than Windows XP like Windows 98 or something, (well for one thing, you might not find it under manage....) select FAT 32 from the list.
Follow the instructions and then restart. Hopefully it'll now recognise it.