There’s a difference between “Operating System not found” and “Boot device not found”. With the first a file like the bootmgr file could be missing, with the second the BIOS isn’t configured to look at the SSD before the HDD.
And depending on the system the BIOS may have to be tricked into finding the removable device. On my netbook I had to make my USB drive the 2nd removable device before the machine would accept it. USB Floppy, USB SanDisk, HDD.
