MacworldAt a glanceExpert's RatingProsFastest external enclosure outside of Thunderbolt 5Super affordable for 40Gbps USB4Handsome, easy-access design with silicon jacketConsThermal issues under super heavy load with jacket onOur Verdict UGreens solid, good-looking CM850 USB4 enclosure outperforms everything except Thunderbolt 5, and gives that more than a run for its money in real-world transfers. it also does so for significantly less cash.Price When ReviewedThis value will show the geolocated pricing text for product undefinedBest Pricing TodayIf youve shopped Thunderbolt in your quest for fast external Mac storage, youll know that sticker shock is often a pitfall of the process. You can reduce the monetary jolt by switching your search to USB4, which, being an outgrowth of Intels PCIe over-a-wire technology is just as fast as Thunderbolt 4.And its significantly cheaper than Thunderbolt, especially if you roll your own with something such as the UGreen CM850 NVMe M.2 enclosure.The CM850 is a more affordable version of the Ugreens CM642 that uses the same ASMedia ASM2464PD chip. Its a gun-metal gray aluminum enclosure thats approximately 4.75 by 2.1 by 0.6 inches and weighs 4.4 ounces.Its heft is nice, as is the top panel that pops off to reveal the internal M.2/NVMe slot that supports 2230 (22mm wide, 30mm long) to 2280 SSDs secured by a captive screw. The panel doesnt have small screws to deal with (or lose).The CM850s interior M.2 slot. Note the fan underneath the screw thats used to secure the NVMe SSD.Your options on the Mac for 20Gbps/40Gbps performance are Thunderbolt 3, 4, or 5, all of which involve significantly pricier enclosures, and are not supported on the majority of Windows machines. That can be important if you work in a mixed environment.Adding an NVMe SSD to the CM850 will cost you anywhere from $40 (256GB) to $650 (8TB), with the sweet spot being the 1TB ($60) to 2TB ($160) range. Note that USB4 (or Thunderbolt for that matter) cant use the extra bandwidth provided by PCIe 4.0 or 5.0 SSDs. PCIe 3.0 SSDs are quickly disappearing, but the point is, dont overbuy.Beyond that, avoid QLC SSDs if you regularly write large amounts of data, and buy a DRAM design if youre going to run an operating system off the drive or want top random performance. The best bang for the buck however will be cheaper TLC host memory buffer (HMB) drives.The CM850 with its silicone jacket in place. Artistically lit as wellUgreen CM850: PerformanceThe CM 850 is darn fast. In fact, in the aggregate, its the fastest external SSD weve tested. Take that with several grains of salt, however, as I populated the CM850 with the super-fast 8TB WD850X for my testing.Also, the Thunderbolt 5 OWC Envoy Ultra was significantly faster in Mac synthetic benchmarks, though its real-world transfers were little faster than SSDs with less haughty benchmark results.The charmingly simple, but generally quite accurate Disk Speed Test from Blackmagicdesign shows the 3GBps plus you get both reading and writing with the CM850given a suitably fast NVME SSD inside.The charmingly simple, but generally quite accurate Disk Speed Test from Blackmagicdesign shows the 3GBps plus you get both reading and writing with the CM850 given a suitably fast NVME SSD inside.AmorphousDiskMark (a port of the original CyrstalDiskMark) also shows well over 3GBps transfers.AmorphousDiskMark (a port of the original CyrstalDiskMark) also shows well over 3GBps transfers.ATTO Disk Benchmark says that the CM850 is perfectly capable of utilizing much of the performance any NVMe SSD will deliver. Within the confines of 40Gbps that is. PCIe 4.0 and 5.0 SSDs can go much faster than the 3GBps USB4 and Thunderbolt 4 allow.ATTO Disk Benchmark says that the CM850 is perfectly capable of utilizing much of the performance any NVMe SSD will deliver. Within the confines of 40Gbps that is. PCIe 4.0 and 5.0 SSDs can go much faster than the bus allows.I also ran the CM850 through PCWorlds battery of tests, which is how that fastest claim is substantiated.The following are the results from CrystalDiskMark 8, as well as PCWorlds 48GB transfers and 450GB write. These are all run on the PCWorld storage test beds Thunderbolt 4 ports, not a Mac. Theyre still indicative of superior performance given a superior NVMe SSD inside of course.Overall, the CM850 is number one among all external SSDs tested on that platform, with the CM642 finishing a very close second. That said, the Adata SE920 and OWC Express 1M2 are still both very fast drives that didnt have the uber fast WD 8TB SN850X to help out.The UGreen siblings give their pre-populated rivals more than a run for their money. Keep in mind the super fast WD SN850X was inside. Longer bars are better.In both CrystalDiskMark 8s sequential (above), and random (below) performance tests, the CM850 did very well. The UGreen siblings didnt dominate in the 4K tests, but the results are still very good. Again, the super fast WD SN850X inside helped. Longer bars are better.The CM850, again with the aid of the WD SN850X was the fastest external enclosure weve tested with our 48GB transfers.The CM850 and its cousin bested both their rivals in our 48GB transfers, though the difference is within the margin of error. Shorted bars are better.The CM850 turned in the fastest 450GB write weve seen from an external drive. Bear in mind that the CM642 had the same 8TB WD SN850X inside. The WD SN850X allowed the UGreen enclosures to finish number 1 and 2 in our all time 450GG write times. Shorter bars are better.I did run into an issue with the CM850. While performing the 450GB write in the PCWorld testing, the drive would disappear with the sleeve on. No doubt due to thermal issues, internal fan or no. Removing the silicone jacket allowed it to finish, as did running a fan over it.Other than that, I was absolutely delighted with the CM850s performance. (Who other than PCWorld writes 450GB files on a regular basis?)Should you buy the Ugreen CM850?The CM850 (or its pricier CM642 cousin) is an absolutely fantastic way to add 40Gbps storage to your Mac without paying that heavy Thunderbolt surtax. If you already have an NVMe SSD great, if you dont, buy one and have at it. Good job, Ugreen.