They are different things.

RAM - Random Access Memory - micro chips located on the computer's motherboard. The main processor takes data from a hard drive and temporarily stores it in RAM to work on it. This is because retrieving data from RAM is considerably quicker than retrieving it from a hard drive. Data stored in RAM is lost when the computer is turned off. That is by design.

Hard Disk - A physical disk connected to the motherboard. Data is stored permanently on a hard drive and is not lost when the computer is powered down. The hard drive is the essential data storage device of any Desktop PC. But accessing data from a hard drive is slow compared to accessing it from RAM, so the processor copies as much data as it can from the hard drive into RAM initially so it can be worked on faster.

Why do you thing you are losing gigabytes from your hard drive's free space?

Don't forget, If you're asking for technical help, please be sure to include all your system info, including operating system, model number, and any other specifics related to the problem.