I am copying 300 GB of large files from an SD card to an internal SSD using a USB SD card reader, and the transfer speed fluctuates heavily.

For example, it starts around 80–90 MB/s, then suddenly drops to a few MB/s, sometimes even below 1 MB/s, before jumping back up again. This cycle repeats throughout the transfer, but without any clear frequency and each cycle is rather different.

Details:

  • SD card: SAMSUNG EVO Select Micro SD-Memory-Card, 512GB microSDXC 130MB/s Full HD & 4K UHD, UHS-I, U3, A2, V30.

  • Card reader: SmartQ C307 USB 3.0 Portable Card Reader for SD, SDHC, SDXC, MicroSD, MicroSDHC, MicroSDXC, with Advanced All-in-One Design.

  • Destination: internal NVMe SSD (1WD_BLACK 8TB SN850X NVMe Internal Gaming SSD Solid State Drive - Gen4 PCIe, M.2 2280, Up to 7,200 MB/s - WDS800T2X0E)

  • OS: Windows 11 24H2 Pro.

  • Laptop: MSI GS76 Stealth 11UH (mirror)

  • File information: mostly large files, typically several GB each. Over 300 GB in total. The files are videos created with a GoPro 10. Number of files: 210.

  • I disabled the antivirus.

  • I use File Explorer for the copy.

  • I didn't see any other process clogging disk read/write:

What causes this kind of extreme speed fluctuation? Are there ways to achieve more stable transfer speeds when copying from SD cards?

https://superuser.com/a/1782396/116475 blames cache but why the copy can't be done directly to the SSD, why would caching be such a bottleneck and are there ways to achieve more stable transfer speeds when copying from SD cards?

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    parent

  • Bus speed, memory, processing speed

    parent

    Thanks, the transfer speed starts around 80–90 MB/s, then suddenly drops to a few MB/s, sometimes even below 1 MB/s, before jumping back up again. Can bus speed, memory, and/or processing speed explain such a drop in speed transfer?

    parent root

  • Part of it will be file types; dome are denser than others. Also, if copying from USB to internal SSD, Windows id probably AV scanning them.

    parent

    Thanks:

    • File information: mostly large files, typically several GB each. Over 300 GB in total. The files are videos created with a GoPro 10. Number of files: 210.

    • I disabled the antivirus.

    parent root