You may be able to locate an older style ATA/IDE 2.5-inch hard disk drive (new) and
partition it, so Mac OS 9.x can run from one section and Mac OS X from another.
Faster spin rate drives are better than slow ones, though not sure how battery life
may fair if you happened across a 7200-RPM ATA/IDE hard disk drive. 5400 is OK.
4500 was stock, at least in my first edition DualUSB white iBook G3 500 05/2001.
A new 'legacy' OWC SSD for older computers is available, they are quick and are
backward compatible for use in ATA/IDE (PATA) configurations... Partitioned it
could support several boot volumes and storage.
http://eshop.macsales.com/shop/SSD/OWC/Mercury_Legacy_Pro
Since the limit in bootable drive size applies to pre-2002 computers with the slower
and smaller controller capacity, a second partition for a booting OS9.x system is a
fair way around the limit imposed by the vintage hardware and early Mac OS 9.x.
This site has some information, though not directly related: (also a few D/L links)
http://www.macos9lives.com/mac%20os%209%20lives_003.htm
Old or vintage information, harder to find certain info:
http://lowendmac.com/
About various drives specifications:
http://lowendmac.com/2014/maximum-hard-drive-size/
"The original IDE specification supported 22-bit logical block addressing, which supported
a maximum drive size of 2.1 GB (2.0 GiB). The first ATA specification increased that to 28-
bit addressing to support drives up to 137 GB (128 GiB). That remained the limitation until
Ultra ATA/100 (a.k.a. ATA-6) arrived with 48-bit addressing in 2002, in theory supporting
drives up to 144 petabytes (128 PiB), although it will be a long, long time before drive of
anywhere near that capacity become a reality." - (from) lowendmac, maximum drive size
So it would appear you may choose some other kind of drive setup, since Mac OS 9.x
is a fairly sparse system and runs well with much less resource demand that OS X...
An external could be used to good advantage to hold copies or clones of system/software.
An external FireWire enclosure (multiple port) such as OWC has may be useful to run 2+
systems in partitions for use with more than one vintage Mac; the FW400 vs FW800 of
some enclosures may not be an issue. If you contact OWC they should be able to help if
you are considering any product, and you need not buy to ask important questions.
Well, the time is about 1:30AM locally and so I'm going offline. Hopefully someone such
as BDAqua may see your post & reply, or another vintage user of early Mac OS. As it is
now, I no longer have a Mac that boots Mac OS9 + OSX. I do have a PM 7600/132 that
is sitting, probably needs a PRAM battery; has OS8.6. Been 10= years since it was run.
With a larger drive in an external enclosure, you could partition it for later useful purposes.
In any event...
Good luck & happy computing! 🙂