Extremely slow copies from external drive on M3 MacBook Pro but not Intel MacBook Pro

I recently got a 14" MacBook Pro (M3 Pro, running macOS 14.3.1) and I've been trying to transfer my files over. I tried using Target Disk Mode (with two different cables, one USB-C and the other a certified 2ft Thunderbolt 3 cable from a Sonnet eGPU) however the M3 Mac's drive does not show up on the Intel Mac (2018 MacBook Pro running macOS 13.5.4). I tried everything and then eventually gave up. So instead I'm copying everything to an external drive (which is a Samsung 970 EVO NVMe, encrypted, APFS) in a Thunderbolt 3 enclosure, using the same Thunderbolt 3 cable above. When I copy everything from the Intel Mac to the drive it copies lightning fast. However, when I plug it into the M3 Mac and try to copy, it is extremely slow. Over the past hour it has transferred (13,000 files in total with a size of 7.44 GB) at around **800KB/sec** according to my math, and still estimates 2 hours to go. Copying these files onto the drive from the Intel Mac must have taken less than a minute. Any idea what's going on here? I will note that this is the first time this drive has been connected to this new Mac, if that could possibly have anything to do with it. This is a pretty fresh Mac, I only started setting it up yesterday; I've loaded Homebrew and some packages, made some shell modifications, added a few apps (Firefox, Visual Studio Code) and that's it. EDIT: The problem exists with other external drives as well. I have an external SATA SSD in a USB-A 3.0 case, using a standard USB-A to USB-A cable with an Amazon Basics USB-A to USB-C adapter. For a 2.95GB transfer the first 900MB goes in about 15 seconds, then slows to crawl, taking a couple minutes to reach 1.21 GB, then a couple more to reach 1.54GB. The progress bar also stops moving and counting slowly, it just jumps between these values: two minutes after reaching 900MB: two minutes later: **UPDATE:** So now I've spent all day with this. It passed Apple Offline Diagnostics, and I turned off Spotlight indexing for the external drive (it would have already been indexed by my other Mac) and booted in Safe Mode. Same thing. Transfers take a while to "warm up", i.e. the stripe in the status bar moves from side to side for a bit, then it usually gets the first 10% or so pretty quick, with frequent updates. Then it slows to a crawl, first estimating 1 minute, then more and more depending on the size of the transfer. The CPU load is minimal and I didn't see any Spotlight processes running as far as I'm aware. The files get there, and there don't seem to be any problems with them, but... There does seem to be a glitchy/laggy interaction with the drive (on the M3 Mac), it doesn't seem to want to eject readily, saying it's in use by some other process. Cancelling a transfer might not work, and opening the drive in the Finder might take a long time to view the contents. Also, watching a high-defintion video from the external drive is almost seamless and I can scan forward and reverse easily. But according to BlackMagic Disk Speed test I only get around 2.2 MB/sec from the drive. Just like the file transfers it starts at 0 for a while, goes up to 4 after a minute, goes down to this after another minute: So there is something strange going on... **UPDATE #2:** I'm thinking there's something wrong with this Mac. This can't be normal. I have a 240MB TIFF, 14787x5993 pixels. Opening in Preview on the 2018 13" MBP beach-balls for 5 seconds, but then I can zoom in with Cmd+0 and scroll around right away. On the fresh 2023 14" MBP it opens right away, but then if I zoom with Cmd+0 it beach-balls for at least 25 seconds before it slowly "unblurs" the image to its actual resolution. Scrolling around within the image is very laggy. This is with the 2023 running nothing and the 2018 running multiple applications, and there's nothing eating up CPU in the background according to Activity Monitor.