Before you buy: what free slots do you have
Adding storage is only simple if you buy a drive that matches an empty slot. So the first step happens before you spend anything. Shut the PC down, unplug it, open the side panel, and look for three things.
M.2 slots. These are short horizontal slots on the motherboard, often tucked under a metal heatsink. Your boot drive probably sits in one already. Most boards have at least one more. Check the motherboard manual for how many there are and where they are.
2.5 inch drive bays. Small trays or mounting points, usually behind the motherboard tray or on the floor of the case. These take flat SATA drives about the size of a deck of cards.
3.5 inch drive bays. Larger cages near the front or bottom of the case. These hold mechanical hard drives.
While you are in there, look at the power supply cables. A SATA drive needs a flat SATA power connector running to it. Most supplies have a spare on the same cable chain. If yours does not, you will need a modular cable or an adapter.
Which type of drive to add
An M.2 NVMe drive is the fastest option and the tidiest. It screws flat onto the board with no cables at all. If you have a free M.2 slot, this is usually the best choice for games and programs.
A 2.5 inch SATA drive is slower than NVMe but still far quicker than a hard drive, and it is a good pick when your M.2 slots are full or your board only has one. It needs a data cable and a power cable.
A 3.5 inch hard drive gives you the most space for the least money and is the right call for large libraries you do not need to load fast: recordings, backups, video files, older games you rarely touch. It is the slowest option by a wide margin.
A quick note on M.2 slots. Some run at full NVMe speed, others are wired for SATA drives only, and a few share lanes with other slots. The manual spells this out. Check it so you buy a drive the slot can actually run.
Installing an M.2 NVMe drive
Work on a hard surface, and touch a bare metal part of the case first to discharge static.
Find the empty M.2 slot. If it has a heatsink cover, remove the small screw and lift it off. Note the little standoff or latch at the far end of the slot, this is what holds the drive down.
Slide the drive into the slot at a shallow angle, gold contacts first, until it seats. It will stick up at roughly thirty degrees. Press the free end down flat and secure it with the tiny screw, or click the latch across it if the board uses a toolless design.
If the slot came with a thermal pad under the heatsink, peel the plastic film off it before you put the cover back. Screw the heatsink back down. Close the case, plug in, and boot.
Installing a 2.5 or 3.5 inch SATA drive
Mount the drive first. A 2.5 inch drive goes on its tray or screws to its mounting point. A 3.5 inch drive slides into the cage and locks in with a tray or screws. Get it firmly held so it cannot vibrate.
Run a SATA data cable from the drive to any free SATA port on the motherboard. The ports are usually a block of flat connectors near the edge of the board, often numbered. The cable and ports are shaped so they only fit one way.
Run a SATA power connector from the power supply to the drive. This is the wider flat plug. Again it only fits one way.
Double check both plugs are fully home, then close the case and boot.
Setting up the new drive in Windows
A brand new drive will not appear in File Explorer yet because it has no partition. That is normal.
Right click the Start button and open Disk Management. When it opens it will usually prompt you to initialise the new disk. Choose GPT as the partition style. If it does not prompt, find the new disk in the lower panel, right click it, and choose Initialise Disk.
Now right click the black unallocated space next to that disk and choose New Simple Volume. Accept the full size, assign a drive letter, and format it as NTFS with a quick format. Give it a label you will recognise, something like Games or Storage.
Once that finishes, the drive shows up in File Explorer and is ready to use.
Moving games and files across
You do not have to reinstall games to move them. Steam lets you add the new drive as a library folder under Settings, then move an installed game to it with a right click, Properties, Installed Files, Move Install Folder. Other launchers have a similar option. The files copy across and the launcher updates its own records.
For documents, photos, and video, a normal cut and paste in File Explorer is fine. If you want Windows to save new downloads or screenshots to the new drive automatically, you can point the default save locations there in Settings under System, then Storage, then Advanced storage settings.
Leave your Windows install and your most used programs on the boot drive. The goal is to move the bulk that does not need to be on the fastest drive, not to scatter your system across two disks.
How much space to buy
Buy more than you think you need. Drives fill up faster than people expect, and adding a second drive later is more hassle than sizing the first one right. A modern game install can run to well over a hundred gigabytes on its own, patches keep growing them, and video capture eats space quickly.
For a games and programs drive, aim for enough room to hold everything you play in a given month with plenty to spare, so you are not uninstalling things every time you want to try something new. For a bulk storage drive holding recordings, backups, and an archive of older games, go as large as the budget allows, since the cost per gigabyte drops as the drives get bigger.
One more point on drive health. Solid state drives have a write endurance rating, but for normal home and gaming use you will replace the drive for being too small long before you wear it out. Mechanical hard drives are the opposite: plenty of space, but moving parts that fail with age, so anything you cannot afford to lose should also exist as a second copy somewhere else.
The bottom line
Adding storage is one of the safest upgrades you can do. Nothing gets removed, nothing gets overwritten, and if you buy the wrong shape you just return it. Check your free slots before you buy, match the drive to the slot, mount it firmly, and set it up in Disk Management. Twenty minutes of work and the out of space warnings are gone for a long time.