Can't see start up disk

G5, 10.5.8

I've got my HD bootably cloned with superduper on two separate connected external drives. I can see them mounted on my desktop and SD can still clone to them.

However I went to boot into one of them using the option key method, but I could not.

So I checked the start up disk option on the preferences and for some reason the mac will not see either of the bootable clones, which it always has up until now and I do not know why.

In the disk start up window there is a network device with a ? mark on it but both the HDs with the bootable clone are plugged in directly to my mac.

I'm at a loss as to what the reason could be for the non appearance of the clones.

Any suggestions appreciated.

These are both firewire and apple partition mapped, but as I said, I've been able to boot into these for a couple of years no problem. In fact only a few weeks ago was the last time. Everything else seems fine, as I said, they mount and update. They just are not showing up to boot from? Can't understand this at all. Makes me a little nervous because I thought I'd be covered in case my HD packs in what with two separate bootable clones on different drives. But now that suddenly I can't boot into them, I feel like I'm in a dream downtown and nekkid.

EDIT: I had the external drives daisy chained into one fw socket but I have now just plugged each one directly into the mac and still no go.

EDIT: Strangely I just checked the partition maps and one drive is an APM and the other is MBR although I've been able to boot into both of them in the past.

EDIT: it's back. Well I shut everything down yet again including both drives and went for a walk. Upon my return I started up with the option key and lo and behold both clones showed up. However when I went to look at the Startup disk app, only the clone with the APM was showing. So just for the hell of it I restarted and was able to boot into the other MBR clone even though it was not showing in the startup disk. So I'm not sure what all this was about but it's fixed itself.