Hm.. just my two cents.
why not keep the OS where it is and migrate all other files to new HDD(s)?
Games don't need to be reinstalled...just copy and paste them over to new HDD and create a shortcut from the games EXE file to the desktop. Easy as pie. And theres no need to worry about file transfer speeds between internal hard drives as it's many times faster in comparison to games downloaded from cloud services such as Steam or Origin.
BEWARE: Some games will work if their installation folder is simply moved, rather than a full un/reinstall, but many will not! It greatly depends on whether or not the game has registry entries that specify file locations. Now, you can search the registry for the game's folder name, to find any keys that need to be changed, but I definitely do
not recommend this unless you damn well know what you are doing;
you can quite literally break Windows with a bad regedit. Just try each moved game; if it works, it doesn't need a regedit. If a regedit doesn't fix a game, it's very likely an .ini or some other ancillary file in the game folder also has a reference to the old location.
Some lower grade internal HDDs are 5400rpm as well, fyi.
Such as WD Greens.
If you want SSD performance with HD capacity, you may want to check out the
Seagate Momentus hybrid SSD/HDs (Note: the hybrid version only comes in the 500 GB size currently, AFAIK).