Do you even need an install disk?
The last time I installed workbench I booted the floppy, formatted the hard disk, then simply copied the contents of the workbench copy onto the hard disk and (I think) moutned the drive.
Hey presto, one bootable hard disk. No need for install disk.
You need at least som tool to create the partition table and create bootable partition, this is normally HDToolBox that is on the Install diskette.
If your disk is already partitioned with RDB and has ticked off a bootable partition, then fine. An alternative is to use for example HDInstTools (on aminet) instead of HDToolBox.