One of the most useful - and I believe coolest - features of the Mac
OS is the amazing flexibility of the operating system. Specifically, I
am referring to the ability of the OS to be copied from one computer to
another without a need for reinstall.
Compared to Windows, the Mac OS is like a double jointed
gymnast.
One Installation, Many Macs
Something that most people may not realize is that an installed copy
of the Mac OS can be almost endlessly copied and instantly run on any
Mac for which there is basic hardware support (e.g. you can't clone
Panther to an Intel or a newer PowerPC Mac, because you would get a
kernel panic on boot due to a lack of support for the newer
hardware).
This principle forms the basis of Low End Mac's Unsupported Leopard page. If I so
desired, I could buy a copy of Leopard and install it on an external
drive using my fairly new Intel-based iMac. I could then attach my
eMac in Target Disk
Mode and clone the Leopard installation to it using Carbon Copy Cloner or
SuperDuper.
The only reason I would even have to use the external drive would be
to get around the GPT (GUID
Partition Table)/APM (Apple
Partition Map) formatting problem. Intel Macs can only install the Mac
OS to GPT drives, although they can boot from either GPT or APM drives.
PowerPC Macs can only boot from APM drives. Thus the clone is necessary
to move the copy of Leopard onto an APM drive.
My 700 MHz eMac doesn't even meet the minimum requirements for
Leopard; it lacks 167 MHz of processor speed and 192 MB of RAM. The
only thing that matters in this scenario is the G4 compatibility
compiled into Leopard.
This procedure also works with the Classic Mac OS, though I find one
must be more careful with version numbers than in OS X. Through
time I ended up with a clamshell
iBook and CRT iMac and only
one set of OS 9 discs for an even older clamshell iBook that no
longer functioned. I could install the system on the iMac, as it was
the only one that could boot from the discs. Then, after updating the
system to 9.2.2, I would copy the contents to the iBook, which would
work flawlessly.
Unfortunately, when I tried to clone the system to my eMac, I ended
up with no sound or 3D graphics. I suspect that the problem lies in an
old ROM file in the System Folder that predates the eMac.
Well That Didn't Work
From experiments and experience, I have learned that this feat is
impossible to do on Windows. Believe me, I have wasted many
hours trying to get it to work. My attempt was simple: I was moving a
Parallels system to the recently released VMware Fusion for my math
teacher. I followed VMware's instructions to the letter. My new image
booted fine, though it complained of needing reactivation. However, I
had to do a repair install of Windows to change to an ACPI
Multiprocessor HAL.
This was necessary for two reasons. First, one of the reasons my
teacher moved to VMware was its support for two virtual processors.
Second, the virtual machine couldn't automatically shut down because
Parallels virtual machines used the older APM HAL.
After I performed the process, the system developed several massive
bugs. For example, Windows Update simply ceased to function, leaving
security vulnerabilities. After spending hours trying to fix this, I
eventually ended up with a non-system disk, which for those unfamiliar
with Windows means that the system will not boot. At that point I
turned to my teacher and exclaimed, "Well that didn't work!"
In the end, I ended up reinstalling Windows.
Come on, Microsoft. You can move an installed copy of Mac OS X
between two different processor architectures and it won't bat an eye.
Your OS is far too rigid and brittle.
Apple Bailed Me Out
If it wasn't for this feature, I would have been in deep trouble
many times.
When I once managed to install the core system for an Intel Mac onto
my eMac, generating an unbootable system, my Panther iBook saved me. I
was able to boot my iBook in Target Disk mode and boot my eMac from the
iBook's hard drive until I could get it fixed. Thankfully, the
fantastic support people of the Apple Store's Genius Bar were kind
enough to reinstall Tiger for free. As an aside, make sure and
generously thank the Genius Bar staff when they fix your problems.
Trust me, they deserve it.
If Apple were to integrate some kind of copy protection, such as
what has been rumored, scads of people wouldn't be able to use yet
another feature that makes the Mac superior. 
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