Wirebook › Bonus lesson
Why is copying to a USB stick so slow?
You drag a folder onto a USB stick and the bar crawls. Sometimes it races for a few seconds and then slows to a dribble. Sometimes a small folder takes longer than a whole film. To see why, imagine a line of ants carrying crumbs (the pieces of your files) from your laptop, over a bridge (the USB port and cable), into an anthill (the stick's memory chips). The copy can only go as fast as the slowest part, and there are three usual suspects: a skinny bridge, slow or tired ants, or too much paperwork at the gate. And at the end, a game: catch a stick that lies about its size.
Suspect 1: a skinny bridge
The bridge is the USB port on your laptop plus the cable. Old USB 2 ports and cables are a narrow rope bridge: the box says 480 megabits a second, but that's bits, and once you divide by 8 and take off the overheads, real copies top out at about 35 to 40 megabytes a second. USB 5 Gbps (often called USB 3) is a wide road bridge, around 400 MB/s in real life, and 10 Gbps is a motorway. If you plug a super-fast stick into a USB 2 port, or use an old charging cable that only has USB 2 wires inside, the ants queue at the bridge no matter how quick they are. Blue USB ports, or ones marked SS (SuperSpeed), are usually the faster kind.
Suspect 2: tired ants
The ants are the memory chips inside the stick, and writing to them is slower than reading. Many sticks and SSDs carry a snack bag: a small part of the chips used in a quick mode, called an SLC cache. While the snack bag lasts, the ants sprint. When it runs out, every crumb has to be packed the slow, careful way, and the speed can drop to a quarter or less. That's the race-then-crawl you see on big copies. Cheap sticks have tiny snack bags and slow ants, so a big film crawls at under 10 MB/s even on a perfect bridge. A spinning hard drive has no snack bag at all: steady ants that never sprint and never get tired.
Suspect 3: paperwork at the gate
Every file, however small, needs paperwork: the computer has to make an entry in the stick's file list saying its name, its size and where its pieces live, and the stick has to update that list. For one big film that's one stamp. For 20,000 tiny files it's 20,000 stamps, so the ants spend more time queueing at the gate than walking. That's why a folder of little files (a game, a code project, a pile of documents) can take longer than a film ten times its size. Zipping them into one file first turns 20,000 stamps into one.
So, is it tiny ants or a skinny bridge?
Either can be, and that's the whole trick: a copy is a chain, and the chain goes at the speed of its slowest link. Engineers call the slowest link the bottleneck, like the narrow neck of a bottle that decides how fast it pours however big the bottle is. Making any other part faster does nothing at all. A faster port won't help a cheap stick, a new stick won't help on a USB 2 port, and neither will help with 20,000 tiny files. Find the bottleneck first, then fix that one thing.
A few things that really help: plug into a fast port directly, not through a hub or keyboard; use the cable that came with the drive; zip folders of small files first; for big films choose a stick that says it writes fast (the big number on the packet is usually the read speed); and don't copy two big things at once, because the ants then take turns. And on Windows and Macs, the first few seconds always look fastest, because the computer fills its own memory first and the stick catches up later. That's also why you should press Eject and wait before pulling it out.
The stick that lies about its size
Here's a sneaky one. Some very cheap sticks (often "2 TB for a tenner" bargains) are fakes: the little chip that runs the stick, called the controller, has been reprogrammed to tell your computer it's much bigger than it really is. Your computer believes it, shows lots of free space, and happily copies to it. Everything looks fine until you open the files later and find they're broken. There are two kinds of liar. A loop-around liar runs out of real shelves and quietly goes back to the first one, so new files squash old ones. A black-hole liar just drops everything that doesn't fit and pretends it saved it.
You can't catch a liar by looking at the size, because the size is the lie. You catch it the way a teacher checks homework: fill it right to the top with numbered marbles, then read every single one back and check the numbers.
Why only a full test works
Did you spot the trick? If you only fill 8 marbles into a stick with 16 real shelves, every marble comes back perfect, and the liar gets away with it. A liar only shows itself once you write past its real size. That's why the real tools grown-ups use, H2testw on Windows and f3 (f3write and f3read) on a Mac or Linux, fill the whole stick with numbered test data and then read all of it back. The point where the numbers start going wrong tells you the real size. If a stick fails, don't use it for anything you care about: send it back for a refund and keep the test result as proof. And remember the number you can trust is the one that comes back right, not the one on the label.
Related lessons: how a download picks up where it left off · why zipping photos barely makes them smaller · why the game lags when someone downloads · is my hard drive getting old?.
Grown-up twin: Tooltern's help pages on why a USB drive is slow or the speed drops, how to test for a fake-capacity stick or SD card and which format a USB stick needs for a TV, car or console, plus the unit converter for megabits versus megabytes.