You can actually.

Basically, clone your current internal hard disk into your external drive (firewire) using any good backup software like SuperDuper! or something. After that's done, test that you can boot OS X off the external before wiping out your internal HDD to install Tiger. If all goes well, you're fine and good.

If you want to do the internal HDD exchange, bear in mind it's not easy to DIY, and if you scratch any part inside your mac while exchanging the HDD, you may void your warranty. If you don't want to void your warranty and don't trust yourself to do a flawless, scratch-free job, take it to any Authorized Service Center like VR Technology and get the techs to do it for you. Won't void warranty, but costs RM100 install fee. Then, take back your original internal HD, put it into an external FW casing and boot off it.

You should be able to see your disks in Disk Utility. However, you can install OS X on the external FW disk if the boot partition scheme for that disk is the correct one for your mac.

For Intel macs - GPT (GUID Partition Table)
For PPC macs - APT (Apple Partition Table)

Use Disk Utility to format them appropriately (and use HFS+ Journaled too). The partition table bit is under "Advanced" in Disk Utility if I remember correctly.

Disk Utility will let you know if your external disk allows OS X to be installed on it or not. If it refuses to let you install on it, reformat using the appropriate partition table for your disk (see above) and try again.

Have not tried/do not know anyone who's done it on USB disks, so can't help you if your external is USB.

Also, you can use Disk Utility to restore your internal OS X to external and it'll basically clone your OS X installation onto the external drive, and then you can install Leopard onto the Internal.

This post has been edited by djspinnet: Oct 24 2007, 12:42 PM

Windows, don't think so, but OS X, yes.