Unable to boot

[phila]

Thanks for your help Dave. The quickness of your replies is greatly appreciated. I sort of thought that if the external HD booted from 'Startup Disk' then the SD side of things was most likely OK.

That last procedure you suggested didn't work so I rang Apple Care and, in a round about way the problem is "fixed". In case you ever need to know for someone else, or care to suggest why this is so:

If I boot into Target Disk mode, then shut down; the next Option/boot shows the d2 disk in the 'Startup Manager' screen - as a third choice (with name). The second disk, which I had assumed was the d2, is to be ignored apparently.

I can then select the d2 and boot from it with no problems.

But I have to do the Target Disk mode procedure all over again for the next time I want to boot from the external drive.

??????????

Thanks again,

Phil