trying to boot at startup from backup

[tier0605]

Hi everyone,

how do you switch from one button to the other on the option screen? I am having the same problem where my external usb hard drive doesnt show up as an option to reboot when the option screen comes up, but my other problem is that I am unable to navigate around the option screen at all. The options that show up are the reload button, Macintosh HD and the third button is an arrow pointing to the right (not sure what that is?), but I dont know how to navigate around them. I tried the tab button but I dont see any one of these buttons being highlighted.
The external hd I am using to try and reboot is an iomega usb 2.0 on my imac g5 but no success so far.
I am a little wry about switching my startup disk in my system preferences to my external hd because I am worried that if the system doesnt start up or it has some problem, then I have no way of accessing the system preferences again and changing the startup disk to the Machintosh HD and I would have to use Apple's rebot disk.