Hello how are you? Normally, in the world drive C is considard as deafult drive. Actually it is preconfigured in the BIOS to first check the C: drive for Operating System after POST. If it finds any OS then it transfer the conrole to the OS. On the other hand, if it doen't find aany OS on C: then it will check other drives for OS.
You cannot change this option from BIOS(may be some BIOS also have this option). BIOS will always check C: drive.
When you are installing the operating sysem e.g windows, it ask you to select drive where you want to install the OS or windows. Defaul is C: but you can also chose different drive right there.
Is it enough info....
Take care of you and others and be happy...
Bye...
Dur-E-Wahab