Corsair EX400U review: Game-changing USB4 speed in a magnetic, phone-friendly form factor

Over the last couple of years, the vast majority of external SSDs to hit the market have been 10Gbps or 20Gbps USB 3.2 drives, effectively capped at just over 1,000 MB/s or 2,000 MB/s respectively. But here in 2025, that cap has been blown off, as we’ve recently tested LaCie’s first Thunderbolt 5 drive (rated at 6,700 MB/s reads), as well as Corsair’s EX400U, a USB4 drive that’s rated to 4,000 MB/s read and 3,600 MB/s write.

As both a reviewer and a user of these drives, I find it refreshing to see new levels of speed and innovation. And as an owner of a Samsung S25 Ultra, it’s also nice to see that Corsair’s drive includes a magnetic ring for snapping it on the back of a MagSafe- or Qi2-compatible phone for easy direct-to-drive video recording or transferring large amounts of files.

The Corsair EX400U blows away the USB 3-based competition, as we’d expect. But on our Windows 11-based testbed it also mostly outperforms the LaCie drive running via Thunderbolt 4. Sure, the LaCie drive is much faster over Thunderbolt 5, but there are barely any Thunderbolt 5-based Windows systems at this point, and not a lot of Macs with that interface either. Even the brand-new MacBook Air only ships with Thunderbolt 4.

So for now at least, USB4 seems to be the sweet spot for external storage, and Corsair’s EX400U delivers great USB4 performance in a compact, magnetic-backed package that’s also much less expensive than LaCie’s TB5 drive. It’s got a good shot of landing on our Best External SSD page, but first let’s take a look at the drive’s specs, from Corsair.

Corsair EX400U Specifications

Compatibility complications

Design and accessories

Corsair’s EX400U is a fairly compact (2.53 x 2.52 x 0.47 inches) silver square with a silver metal top and plastic sides. The company did a good job matching the color and texture of the mixed materials that it used here, but there’s no denying the abundance of seams around the top and sides.

The drive feels solid, with no discernable flex, but there’s no denying that something like SK hynix Beetle X31 looks and feels more premium.

The real stand-out exterior feature of the EX400U is the built-in magnetic ring on its base, designed to let the drive snap onto the back of MagSafe-equipped iPhones and Qi2 Android devices. We’ve seen similar form factors before Lexar, MSI, and others. And I’m sure we’ll see more since roomy, speedy and convenient external storage is great for content creators who are always recording video. But Corsair’s EX400U is the first USB4 drive we’ve seen in this form factor. It certainly won’t be the last.

The only physical accessory included in the box is a 30 cm (nearly 12 inches) USB 4 Type-C to Type-C cable. The length is fairly standard, but it’s also both long for connecting to a smartphone and short for desktops, where the USB4 ports live at the back of the case. At least the magnet on the drive means you can stick the drive on any steel elements of the case so it isn’t dangling while you transfer files.

Comparison products

At 2.53 x 2.52 x 0.47 inches, the EX400U isn’t particularly small or large for an external SSD, though it’s much smaller than its primary competition in our test group, the LaCie Rugged SSD Pro5 (0.67 x 2.56 x 3.85 inches). When I magnetically attached the Corsair drive on my S25 Ultra phone, one corner did encroach onto the camera bump, but the edge of the drive wasn’t visible at all as I cycled through the various sensors.

In terms of performance, the only drive that can hope to be competitive in our test pool is LaCie’s Rugged SSD Pro5. And that drive is definitely faster if you’re plugged into a Thunderbolt 5-equipped host. But limited to Thunderbolt 4 / USB4 on our newly built external storage testbed, the LaCie drive didn’t perform as well as I expected.

Storage testbed update

Just before testing both the Corsair drive and the LaCie Pro5, we updated our external storage testbed to an AMD Ryzen 7600X-based PC with an Asus ROG Crosshair X870E Hero motherboard. This was done in part because we needed a system with native USB4 support for upcoming drives.

All the drives in the charts below have been freshly re-tested on the new X870E system, with the exception of the final Iometer sustained sequential test, which is less about top speed and more about how long a drive can write before depleting any fast cache. Although both the LaCie and Corsair drives had their Iometer testing done on the updated testbed. We also updated to CrystalDiskMark 8, rather than the older (and non-comparable) version 7 we used on the previous testbed.

Trace Testing - PCMark 10 Storage Benchmark

PCMark 10 is a trace-based benchmark that uses a wide-ranging set of real-world traces from popular applications and everyday tasks to measure the performance of storage devices.

In this first test, the Corsair EX400U landed a distant second to the pricier LaCie Thunderbolt 5 drive, but it was still far faster than any other external drive we’ve tested. Remember, though, that the LaCie drive was limited by the Thunderbolt 4 bandwidth of our testbed. On a Mac (or presumably on one of the few TB5-equipped PCs), the LaCie is significantly faster than the Corsair EX400U, but it also costs much more.

Transfer Rates – DiskBench

In this real-world file transfer, the Corsair drive looks much better against the LaCie. Granted, it’s a couple hundred MB/s slower on reads, but on writes the Corsair drive is more than twice as fast, and far faster than any other external drive we’ve tested to date.

Synthetic Testing CrystalDiskMark

CrystalDiskMark (CDM) is a free and easy-to-run storage benchmarking tool that SSD companies commonly use to assign product performance specifications. It gives us insight into how each device handles different file sizes. We run this test at its default settings.

In this best-case synthetic sequential scenario on our USB5 / Thunderbolt 4 testbed, the Corsair drive arguably looks its best. It edges out the bottlenecked LaCie drive on reads and blows past it on writes by nearly 700 MB/s. And unsurprisingly, the other 10Gbps or 20Gbps drives are much slower.

Small file performance is also similarly impressive for the Corsair drive, putting it on top again, while the LaCie struggled here, landing in third place behind the Adata SE920.

Sustained Write Performance

A drive's rated write specifications are only a piece of the performance picture. Most external SSDs (just like their internal counterparts) implement a write cache, or a fast area of flash, programmed to perform like faster SLC, that absorbs incoming data.

Sustained write speeds often suffer tremendously when the workload saturates the cache and slips into the "native" TLC or QLC flash. We use Iometer to hammer the SSD with sequential writes for 15 minutes to measure the size of the write cache and performance after the cache is saturated.

The Corsair EX400U drive lacks the LaCie’s initial write speed cache north of 3,000 MB/s, but maintains a faster write speed for the duration of the test than the LaCie, typically between 1,600 and 1,700 MB/s, while the LaCie hovered around 1,600 MB/s after its cache was depleted. So while the LaCie drive is much faster for tasks around 50 GB or less, if your write workloads typically involve much larger amounts of data with no downtime for the drive’s cache to recover, the Corsair EX400U might be the better option. The top of Corsair’s drive did get hot during this test, though, while the LaCie was only mildly warm.

Bottom Line

For creators who need the fastest speed possible and who live solely within the Mac ecosystem using Thunderbolt 5-equipped machines, the LaCie Pro5 is faster and easy to recommend (even though it costs more than the Corsair). But that drive’s mixed performance in Windows under Thunderbolt 4 makes it tough to recommend in mixed computing environments, where USB4 is a much more popular option. There are certainly many more USB4-equipped computers available now and launching soon than there are Thunderbolt 5 systems.

And in the USB4 realm, Corsair’s EX400U strikes a good balance between performance and price. It’s certainly not cheap at $189 for the 2TB model we tested, or $349 for the 4TB model. But both are downright affordable compared to the LaCie drive, which was selling for $329 for the 2TB model and $529 for the 4TB model when we wrote this.

The Corsair drive also ships in a 1TB flavor, which the LaCie drive lacks, for around $119. Given Corsair’s drive is much faster and more future-proof than top 20Gbps drives like the Crucial X10 Pro, it makes those older drives hard to argue for, since they still often sell for between $100 and $115 at the 1TB level or $179 for 2TB. For now at least, the Corsair EX400U is one of the best external drives you can buy, with a great price-to-performance ratio and a still-unique magnetic attachment ring, which can be handy on phones and desktop PCs.