It is safer to install the BeOS on a separate hard drive. This is very simple to do, and poses no risk to your Mac data on other drives.
However, if you are sure you know what you are doing, and have the right software tools, it is possible to install the BeOS on the same hard drive as the Mac OS.
If this is an existing hard disk you must first defragment the hard drive with something like Norton Utilities -- if you don't you could lose valuable data!
If this is a new hard disk or you have defragmented it, you can go ahead and use your hard disk formatter to shrink your existing hard drive partition, and then create a second HFS volume. Instructions for doing this should be in your hard drive formatter's manual, and we include the general steps to take in the Advance Installation section of the Installation booklet included with your BeOS package.
When you install the BeOS onto one of these partitions (instead of onto a separate hard drive), the Be installer will reformat that partition to the BeOS format. This will destroy all data on that partition!
You can also destroy all of your data if you re-partition your hard drive. Many drive formatters destroy and then re-create disk partitions, rather than re-sizing existing partitions, which destroys all data on that disk. Be sure to read your software manual!