can't external boot
 

So, all of a sudden (I think I last did it successfully last year), I can't boot to a SuperDuper clone. I restart and hit the OPTION key. Goes straight to the login pane. Uh, what??? My boot disks are mounted, and I can see them in System Prefs>Startup Disk. I reset SMC and PRAM and I still can't boot external. My OPTION key works. Now I've heard that there is some mysterious business about "blessing" an external disk to make it bootable. I've never done it. But maybe I suddenly need to do it for some reason? Is that what's going on here?

Are you using a bluetooth keyboard? If holding down Option doesn't go to the boot menu, Option isn't being recognized at boot time. Your backup isn't involved yet at all...

No, I'm using an old crumb-catcher USB keyboard. And my option key is recognized in other applications.

I fact (and now that I research "bless" a little bit, when I do

I get

That doesn't sound too good, does it?

Though it is true that my backups were going fine. If there were a mount point problem, I guess I wouldn't be able to do backups.

The drive clearly isn't mounted with that mount point (if it's mounted at all)...

Well, I am mystified. My daily scheduled backups are going without a hitch, and I can do an external boot handily by selecting the desired external drive in System Preferences->Startup Disk. I just can't seem to manually provoke an external boot by holding the OPTION key when I restart.

My OPTION key works. I can produce accented characters with it in Word, and I can switch speakers by holding down that key when I pull down the audio icon in the menubar.

Now, this is not obviously a SuperDuper problem, unless SuperDuper is leaving the volume in such a state that it isn't seen when I hold the OPTION key when I boot.

Is your keyboard an Apple keyboard? I really think the problem is the key isn't being recognized at startup. Is it plugged directly into the Mac?

Bingo! It's fixed. And many thanks to you for the clue, Dave.

Here's the deal. You asked if my keyboard was plugged directly into my Mac. No, my keyboard was plugged into a new powered USB hub. Now, I know that such hubs can mess with things, and certainly a Bluetooth keyboard won't be read properly on boot. So I plugged the keyboard DIRECTLY into the Mac. It works! (Just like it used to.)

This is an important thing for people to know about when doing bootable backups. That a USB hub may prevent the OPTION key from opening up the Startup Manager.

ALWAYS ALWAYS ALWAYS plug your keyboard directly into the Mac. That instruction ought to be engraved on Mac keyboards.

So you were exactly right that the OPTION key wasn't getting read at startup. But the question was why it wasn't getting read if it was working at other times!

Because the BIOS, at that point, wasn't iterating through all of the hub devices...

That's interesting. I didn't realize the hub was considered a "device". I always thought of it as an extension cord with multiple sockets. So even an unpowered hub would behave this way?

Absolutely. These aren't passive devices.