(RESOLVED) HELP, cannot boot from clone drive (MP2013)

[Riwam]

I need help. Every reasonable person says that before upgrading to High Sierra a bootable clone drive of the previous OS should be made.
However presently, for reasons I don't understand, all my previous clones and any new one are not seen to boot from them with the option key.
I have tried to clone Sierra with CarbonCopyCloner and with SuperDuper!, in both cases with the latest versions.
The cloning in an external USB drive finishes with success. At least it looks like it.
However when I try to boot from that external drive (a USB connected HD) the option key only offers me the internal HD, Bootcamp Windows or the Recovery Partition of Sierra as options.
Previous clones with ElCapitan or with Sierra in connected USB drives are now not offered any longer to boot from them nor new clones presently made.
The Mac is a MacPro late 2013 and the internal SSD shares Sierra and Windows 10.
Any help and hint how to solve this will be very appreciated!!!
THANKS!

ONE MORE STRANGE THING
I found an old Yosemite in an USB drive and THAT booted the Mac.
Since ElCap and Sierra clones do not boot it, could it have to do with SIP which was not yet in Yosemite???

 

[dsemf]

I need help. Every reasonable person says that before upgrading to High Sierra a bootable clone drive of the previous OS should be made.
However presently, for reasons I don't understand, all my previous clones and any new one are not seen to boot from them with the option key.
I have tried to clone Sierra with CarbonCopyCloner and with SuperDuper!, in both cases with the latest versions.
The cloning in an external USB drive finishes with success. At least it looks like it.
However when I try to boot from that external drive (a USB connected HD) the option key only offers me the internal HD, Bootcamp Windows or the Recovery Partition of Sierra as options.
Previous clones with ElCapitan or with Sierra in connected USB drives are now not offered any longer to boot from them nor new clones presently made.
The Mac is a MacPro late 2013 and the internal SSD shares Sierra and Windows 10.
Any help and hint how to solve this will be very appreciated!!!
THANKS!

ONE MORE STRANGE THING
I found an old Yosemite in an USB drive and THAT booted the Mac.
Since ElCap and Sierra clones do not boot it, could it have to do with SIP which was not yet in Yosemite???

I have seen this problem a couple of times. My solution was to do the option boot without the drive plugged in. When the selection list was displayed, I would plug in the USB drive and then it would be added to the selection list.

DS

 

[Riwam]

I have seen this problem a couple of times. My solution was to do the option boot without the drive plugged in. When the selection list was displayed, I would plug in the USB drive and then it would be added to the selection list.

DS

Thank you very much for your very helpful hint.
Before reading it, I happen to do exactly that...without much thought, just trying anything... and as you say, the external clone drive then unexpectedly popped up among the boot possibilities.
It's interesting, if one believes in telepathy (who knows?), that I found exactly the solution you indicated me before knowing it.
I have no idea why it resolves my problem... but the main thing is that it works.
Thank you very much indeed.
I am very grateful to you!

That always remains me of what Albert Einstein is quoted to have once said about the team work in his laboratory:
"Theory is when we know everything but nothing works"
"Practice is when everything works but nobody knows why"
"In out team we have succeeded to join Theory to Practice"
"Nothing works and nobody knows why"

 

[dsemf]

Thank you very much for your very helpful hint.
Before reading it, I happen to do exactly that...without much thought, just trying anything... and as you say, the external clone drive then unexpectedly popped up among the boot possibilities.
It's interesting, if one believes in telepathy (who knows?), that I found exactly the solution you indicated me before knowing it.
I have no idea why it resolves my problem... but the main thing is that it works.
Thank you very much indeed.
I am very grateful to you!

That always remains me of what Albert Einstein is quoted to have once said about the team work in his laboratory:
"Theory is when we know everything but nothing works"
"Practice is when everything works but nobody knows why"
"In out team we have succeeded to join Theory to Practice"
"Nothing works and nobody knows why"

My theory is that the external drive does not become visible fast enough during the option list construction phase. This is probably related to the drive's on-board power up and activation logic.

DS

 

[Riwam]

My theory is that the external drive does not become visible fast enough during the option list construction phase. This is probably related to the drive's on-board power up and activation logic.

DS

If you think that is the explanation, I do not understand why an already powered up and connected external bootable drive does not show in the option list and a later powered up and connected does.
The reaction time is longer in the second case.
And why an already powered up and connected Yosemite external drive did appear in my boot menu as soon as the option tab was activated while external bootable drives with ElCap or Sierra did not.
The OS Sierra has inside a problem IMHO.
Or my MP late 2013 has one.
Or both of them together????