In general, PPC Macs are not bootable from an external USB drive, so the installer may be telling you that by its refusal. There are some exceptions, but functionally fewer than even Apple's cheery article on booting PPC Macs from USB devices claims:
In practice (there have been a lot of sharp people here who've tested it), most PPC externals are simply not bootable. It may be that, with the right controller chipset in the external enclosure, they would be but, the real world, there are pitifully few PPC-bootable USB drives.
You need an external Firewire drive to boot a PowerPC Mac computer.
I recommend you do a google search on any external harddrive you are looking at.
I bought a low cost external drive enclosure. When I started having trouble with it, I did a google search and found a lot of complaints about the drive enclosure. I ended up buying a new drive enclosure. On my second go around, I decided to buy a drive enclosure with a good history of working with Macs. The chip set seems to be the key ingredient. The Oxford line of chips seems to be good. I got the Oxford 911.
Has everything interface:
FireWire 800/400 + USB2, + eSATA 'Quad Interface'
save a little money interface:
FireWire 400 + USB 2.0
This web page lists both external harddrive types. You may need to scroll to the right to see both.
The latest the hard drive enclosures support the newer serial ata drives. The drive and closure that I list supports only older parallel ata.
Here is an external hd enclosure.
Here is what one contributor recommended:
Folks in these Mac forums recommend LaCie, OWC or G-Tech.
Here is a list of recommended drives:
Thanks. This is my first time using FireWire.
How do I cinfirm I am buying the proper cable for my iMac and drive?
I bought a Seagate FreeAgent GoFlex. It lists as being Firwire 800 compatible.
Do I need to make sure the connectors match on both ends?
Or do I just need to match on the input into the iMac?
There are two firewire speeds: 400 & 800. You going to have to figure out what is the speed of you iMac's firewire port.
The plugs look like such:
There are converter plugs from 800 to 400.
Note: there are a some firewire drives that are not bootable.
I think that you are going to have to try it.
1) Does the external harddrive support booting over firewire?
2) Does the firewire port supply enough power for the external harddrive?
There is a four wire Firewire 400 connections. See picture of red cable.
I'm not an Electrical Engineer, so I don't know about these power things.
If you want to boot from it, the Seagate needs it's own power supply - it can't rely on the FW bus for power.
When you buy a FW cable, the pack will tell you what the wiring is - 4, 6 or 9 pin (9 is for FW800 only)
Assuming the Seagate has a FW800 port, and your Mac a FW400 port, you want either a FW800 to 400, 6-pin to 6-pin, or FW800 to 400, 9-pin to 6-pin adapter cable.
Firewire is also known as EEE1394 (as mentioned in the above article).