I ignored the one SSD rule everyone swears by, and the biggest problem wasn't what I expected

Published Oct 27, 2026, 7:30 AM EDT Monica J. White is a journalist with over a decade of experience in covering technology. She built her first PC nearly 20 years ago, and she has since built and tested dozens of PCs. PC hardware is her main beat, and graphics cards and the GPU market at large are her main area of interest, but she has written thousands of articles covering everything related to PCs, laptops, handhelds, and peripherals. From GPUs and CPUs to headsets and software, Monica's always willing to geek out over all things related to computing. Outside of her work with How-To Geek, Monica contributes to TechRadar, PC Gamer, Tom's Guide, Laptop Mag, SlashGear, Whop, and Digital Trends, among others. Her ultimate goal is to make PC gaming and computing approachable and fun to any audience. Monica spends a lot of time elbow-deep in her PC case, as she's always making upgrades, testing something, or plotting out her next build. She's the go-to tech support person in her immediate circle, so she's never out of things to do. Whenever she has spare time, you'll find her gaming until the early hours and hanging out with her dog. One of the most commonly repeated SSD tips is not to fill your drive all the way up to full. It makes perfect sense, and even manufacturers will tell you to leave 10-20% empty, so you should probably listen to them. So, naturally, I decided to ignore all that advice and torture my poor SSD to see what happens. I filled my SSD in stages instead of jumping straight to 95% I wanted to find out where the trouble actually starts A portable SSD atop a desktop PC. For the purpose of this test, I actually used my main SSD, the Lexar NM790. This is my main drive with Windows and everything else installed on it, and while I do have a secondary drive as well, I wanted to feel the full impact of overfilling an SSD, and where better than where the operating system lives? But instead of just going from the nice, comfy 64% it was sitting at all the way up to 100% in one go, I decided to learn whether there was a point where performance suddenly took a nosedive. So, I did the whole process in stages, and at each stage, I did the same things: ran a game, copied files, and checked how the drive handled everyday tasks. I also kept an eye on temperatures with HWiNFO to make sure I wasn't mistaking thermal throttling for lack of space (and if you're in that situation, remember that a heatsink can be a cheap fix). What surprised me was how little changed at first. Even at 90%, my SSD wasn't suddenly struggling to do its job, and getting to 95% didn't turn it into a glorified hard drive. My benchmark results were only part of the story A fast benchmark score doesn't guarantee fast file transfers I started with CrystalDiskMark, and the results were surprisingly good. At around 90% capacity, my Lexar NM790 managed sequential read speeds of 7,429MB/s and write speeds of 6,639MB/s, all the while other stuff was running in the background, too. That's actually ever-so-slightly above the advertised speeds for this drive, so I have to admit I was pretty happy. Even the random 4K results were respectable, with 60.77MB/s reads and 189.76MB/s writes at Q1T1. Looking at these numbers, my SSD didn't take a hit at all from being filled up; I was getting similar results benchmarking the drive at lower capacities, even weeks ago. So much for the performance disaster I was expecting. However, there's a catch. CrystalDiskMark measures performance using test files of a specified size, and I was using its default 1GiB setting. That's a relatively small workload for a 2TB SSD, even one that's almost full. My Lexar also uses dynamic SLC caching, which lets it temporarily write data much faster than it otherwise could. As free space gets scarce, that cache can shrink, but a quick benchmark might never push it hard enough to reveal the problem. To see whether filling my SSD actually hurt its performance, I needed to give it something more demanding to do. Copying large files put the SSD under real pressure The shrinking SLC cache is where things get interesting To put my SSD through something more demanding, I started copying larger files from my secondary drive. Smaller transfers went smoothly, and even at 95% capacity, the drive was still plenty fast. The hiccup came when I tried to move a much larger batch of files. The transfer started off quickly, but the speed became less consistent as it progressed. This is where that shrinking SLC cache comes into play. My SSD uses part of its TLC NAND as a faster write cache, but the amount available on it can decrease in proportion to how filled up the drive is. But was it a disaster? Nope. No disasters here, but maybe not as peachy as the benchmark results showed. I checked whether games and everyday tasks suffered, too The slowdown that matters is the one you actually notice Benchmarks only tell you numbers, and viewed in a vacuum, they're pretty meaningless. The more important question to me was: does it actually slow down your PC if your SSD is an inch away from being full? So, I played some games to check. I paid attention to loading times, both on loading screens and actually loading assets. To stress my PC out even more, I had some browser tabs open, too. The results were underwhelming, but in a good way: everything was largely okay. If anything was wrong, I can't tell you that I noticed it being the case. Maybe I got lucky with my SSD, or maybe the fears are blown out of proportion. On the other hand, this makes sense, because games mostly read data from your SSD, and filling the drive doesn't automatically make its read speeds plummet. One way or another, I was happy with the results. At 95%, free space itself becomes a bigger concern Five percent free doesn't mean the same thing on every SSD One thing this experiment made me realize is that performance isn't necessarily the biggest problem with filling up an SSD. Sometimes, just running out of space is arguably way worse, and that's what I was struggling with. My Lexar had probably close to 100GB available at the end of this test, and in 2026 terms, that's pretty much nothing. I could maybe install one AAA game before I was nearly out. I felt an itch to start deleting stuff willy-nilly, ignoring the 3-2-1 backup rule entirely, just to reclaim some of that space. So, benchmarks aside, that was pretty limiting, because with no space left, even a major Windows update could be a problem. The 20% rule is useful, but it's not a magic number If you came into this article hoping to read all about how my SSD ruined my PC, I'm sorry to have underdelivered. The fact is, nothing much really happened, and I personally didn't see much of a difference between using the drive at 60% and at 95%. With that said, to keep your SSD healthy and let it have all the space it needs for background housekeeping tasks, I recommend listening to the manufacturers, most of which are united in saying you should leave around 20% empty space on your drive.