Automatic reboot vs. hardware (button) boot: wrong partition

Dear all,

I've recently changed my SSD cause it was broken (MacBook Pro mid2010).

I've installed a new one and set up OS high sierra.

Immediatly during the OS installation and ever since, my mac is booting into the correct partion with the button (hardware) and into "prohibited sign" with automatic reboot.

It's like the two commands, for some reasons, were connected to different roots.

I tried to reset SMC/PRAM, check the "start up" partition on settings and other few stuff.

But it remains a mistery. During the updates installation, during the installation of the OS itself, automatic reboot fails. Hardware switch off and on does the deal (= continue and finishing the installation).

Why is this happening ? I have the feeling reboot command tells the mac to restart from a partition is not there. But I have no other partion, I have one disk, formatted with one partition and notheing else (and hardware boot works).

It looks like a windows error, like "mac OS thinks reboot is on A:" but there's no "A:" listed in the booting list.

I am puzzeled.

Please let me know whether something come to your mind, it's not vital, it's just annoying and stupid.

Riccardo