Thanks for the reply.
No, it's not the boot disk.
OS is installed on SSD after ACHI mode was active in BIOS.
So sometimes when my PC boots it hangs after POST, at the ASUS logo.
The Q-code at that moment is A2, which means "IDE detect".
This cannot be caused by the samsung drive being in "IDE mode"?
After a few reboots, my PC returns to run normal and boots up to OS.