so your os is on a sata? if this is the case your optical will be on primary ide on the mobo but the bios may not see it as ide 1 if sata are set for ide sometimes sata set to ide take priority.Do you now have 2 operating systems? does it try to dual boot? Sata drives are always master so it does not matter which one has the os on ive had os's spread all over machines when ive multi booted. As for why its lagging is c: still holding os or is it the old os?
Do you use hdcp off your gfx? if not and its not causing any problems then I wouldnt worry about
I would personally back up all data to whatever drive your gonna want to use as slave and volume label it spare then id open the case and ensure the drive you wish to use as operating system is on sata 0 or sata 1 whichever is the lowest . Then id perform a reinstall of windows to this drive. You wont make a mistake as you set the volume label of the slave so this will show up in set up
