You should do one or the other. Eliminate the 300 and just go with 3 days. or just go with 300.
But 300gb is a ridiculously low number unless you only have a few cams and only recording on motion.
Are you moving to a NAS? If not, best practice is to simply put all the video in the NEW if the drives are all in the computer. Moving them from NEW to STORED is a waste of CPU and HDD usage if all on same drive or even to two different drives in the box.