Booting From an External Hard Drive - Help!

[Mr. Monsieur]

The Short Story:
I created an external bootable drive using Carbon Copy Cloner, but when I restart (holding Alt/Option), the new external bootable drive doesn't appear as one of the options.

The Long Story:
1) The iMac that I'm working on is currently booting from an external hard drive (the internal drive died and, since it's an older computer, I didn't want to spend the money/time replacing the internal HD).
2) The current external HD is older and slower than the one I'm trying to replace it with, which is why I'm trying to replace it.
3) The current external HD is powered through a wall plug, while the newer external HD is powered through the iMac (I'm wondering if this is the issue?) -> the "new" HD is actually an HD I took out of a MBP awhile back when I updated my MBP.

Hopefully that's enough info to work with...*any* thoughts much appreciated!

 

[redheeler]

Is the partition map on the external set to GUID Partition Table?

 

[Mr. Monsieur]

When I setup the drive, I chose the GUID Partition Table option (under Options in the Partition Menu) when I setup the one partition on the drive.
Is there some way I can double-check that this was done properly?

 

[redheeler]

Here's something you can try. On another working Mac, connect the external, go into Disk Utility and hit "Repair Disk" with it selected. There you can also verify that it mounts properly.

 

[Mr. Monsieur]

Thanks so much...I'm not sure why, but that fixed it!