Building a new rig next weekend. Going to install Windows 8.1 via a DVD onto a brand new 1TB HDD while also reusing an old HDD that has Windows 7 on it. I'm afraid the already present OS on the old drive will cause some problems during set up. So would it be okay just not hookup the old drive to the mobo until after 8.1 is up and running? Like hook everything up except the sata to the 7 drive, install 8.1 and all the drivers, shut it down then finally plug in the last sata cable and set boot priority in the bios. I assume this would be fine, just double checking and wondering if there is a better way to do this...
You should be fine, just do as you said and don't hook it up until 8 is installed. When you set boot priority make sure the drive with 8 on it is set first.