if you know the disc make model number etc, some manufacturers have a download available which returns the hardrive to its bought state, from which you have to fdisk and format as normal.
If that program can be run on a slave hardrive without affecting the master i do not know. But it may be an option
not used 98 in many years but with it setup as you have it at present master/slave with 98 as master if you right click on the slave drive in my computer do you get the option to format it?
Although the odds are you have tried it and got the unable to format due in use msg