This kind of problem is very common as I see, but I can't find anything helpful anywhere.
There was a common hdd with an old Win7 installation. A new SDD was bought, a new fresh Win7 was installed on it, dualboot with the old hdd system.
Now, I could see that the new SDD OS needs the Bootloader on the old HDD, but never thought it would actually not work without the HDD.
The HDD broke, now the SDD OS doesn't boot. "BOOTMGR is missing"
I tried loading from a bootable usb flash with command prompt, the sdd is visible, the rebuild of the bootrec /rebuildbcd was successful.
Still the SDD OS doesnt load, it says "no OS found".
I'm out of ideas, can anyone please help me?