Chosen Solution
After replacing the HDD with a SSD, the Mac boots to a white screen, no logo, then displays the folder indicating no OS installed. I have a bootable USB drive with High Sierra installed but holding down option won’t work. I have tried to get to internet recovery but again, screen stays white until folder icon shows. Holding shift does the same thing. No keyboard combination seems to work - always white screen, no apple logo, then flashing folder icon. The Mac worked fine with the old HDD so I doubt there is a hardware fault? I have just left the temp control cable plugged in to the motherboard and the other end is disconnected. I have tried removing the cable completely but that’s hasn’t helped. Any ideas?
Sounds like you installed the system externally. While it will boot externally from any format, it will not do it internally unless formatted GUID Mac OS extended, journaled Also, your drive may be to fast. Tell us what you are using so we can check if it is backward compatible. Your machine has a Storage Interface:Serial ATA (3 Gb/s) that is SATA II
