I want to have to operating systems on my iMac G3. Since I only got a 20GB hard drive, can I buy a FireWire hard drive, and put Mac OS 9 on that hard drive, and put OS X 10.X on the internal hard drive?
iMac, Mac OS X (10.4.11), G3 Processor
Mac OS 9 has a volume-size limit. This old document has information
Macintosh: Using 128 GB or Larger ATA Hard Drives - Apple Support
You are not using an ATA controller for a FireWire external drive, but I believe that 200GB volume-size limit still applies. This LowEndMac article has more information
http://lowendmac.com/2014/how-big-a-drive-does-mac-os-9-support/
TB-sized hard drives were "fantasy land" back when Mac OS 9 was the latest and greatest. Don't go crazy with the hard drive capacity, to use with an iMac G3, just because you can get a multi-TB hard drive these days. The Mac OS 9 system, apps, and related data do not (usually) take up very much space, compared to Mac OS X.
ALSO, to be bootable and accessible with Mac OS 9, the drive must have the Mac OS 9 driver installed. This driver is at the drive level, not volume level. There's an option to install it (a checkbox) in old versions of Disk Utility, when you partition or "erase" the drive.
Yes. If you need to format the drive, you need to install Mac OS 9 drivers on it.
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Installing Mac OS on FireWire Hard Drive?
