"BaseSystem cannot be installed on this computer." That's what the Tiger installer kept telling me.
My Power Mac G4 466 MHz "digital audio" has an external DVD drive connected by USB, so I was attempting to install by directly running the OSInstall.mpkg in System / Installation / Packages, since I can't boot from that DVD drive. I have 2 internal hard drives, with a bootable system on each, so this was feasible by booting into each drive in turn to install Tiger on the other.
Alas, "BaseSystem cannot be installed on this computer."
(Note: this refusal appears to be after the compatibility checks that can be edited in the OSInstall.dist file inside OSInstall.mpkg ("show contents") to allow installation on unapproved machines, such as G3s.)
XPostFacto to the rescue
An open-source project sponsored by Other World Computing, XPostFacto helps to install and boot Mac OS X, Mac OS X Server, and Darwin on some unsupported systems.
But you still need a bootable installer disk, so here's what I did:
1. With Disk Utility, make a disk image of the Tiger install DVD. It doesn't matter where you save it -- as long as there's a few gigabytes (at least 3 GB) of space. (If you're on a network with a DVD-equipped machine, you could do this step on that machine and then copy the disk image to your machine.)
2. With Disk Utility, restore the install DVD disk image to a bootable hard drive (or partition) that doesn't have an OS X system on it already. I copied it to a Firewire-connected external hard drive. Again, you need at least 3 GB of space.
3. Launch XPostFacto. Consider donating to this excellent project. XPostFacto presents a list of disks to install to and a list of bootable installer disks. Pick one of each, and "Install".
XPostFacto will copy some kernel extensions to the target volume and then a modified version of BootX that will use these kernel extensions (along with the standard extensions on the installed disk). Then it will restart the computer and launch (after rebooting in verbose mode) the installer.
4. In the OS X installer, select as the target disk the same disk you selected in XPostFacto.
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