Can't startup from fresh clone - at a loss

[ignoramus]

Darndest thing.

I just did the simplest backup clone of my Leopard 10.5.2 startup drive. All seemed to have gone well, but after selecting the new Startup disk in System Preferences, though it’s highlighted, I’m returned to the original.

The chosen disk, a SAMSUNG HD753LJ, a second internal is formatted HFS+ Journaled.
The computer is a Mac Pro Early 2008.
DiskWarrior OKed it just a couple hours before making the backup.
The original, according to Cmnd + I, is 49.77 GB, whereas the clone is 49.77 GB. The partition to which the copy was made is 200 GB.
The computer was restarted holding down the Shift key before beginning.

Choosing the cloned system after holding down the Option key, returned me to the original disk.
So I thought I’d simply re-do the “Backup - all files” procedure, but found the partition now grayed out.

What’s the remedy? Where do I begin?