I am trying to back up about 250GB into 1T Flash Drive. I tried multiple flash drives and I constantly get the same error: enter image description here

But I am not moving it to my Windows disk. And I am certain I have enough space on the drive. I also tried changing the flash drive type to NTFS and exfat. How can I solve this, and why does it occur? I tried checking checking out multiple similar questions such as: Why a "Not enough space on disk" error on 7zip even with sufficient space? But that didn't help

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    are you copying a lot of small files? exFAT uses very large cluster size by default so the space required would be larger than the total source file size. You must reformat it to a smaller cluster size or use NTFS. I don't think you're correctly formatting it as NTFS
    – phuclv
    Jul 31, 2022 at 15:51
  • @phuclv Yes but why does it matter? If every file is multiplied in size, by 400% max if I remember correctly, then I'll have, worst case, 4 times the total size I am transferring (which is actually 90GB and not 250GB as I previously mentioned). And yes I am 100% certain I formatted the flash drive to NTFS. I used a number of tutorials and did so using cmd, and via right-click on the flash drive (then format & change to NTFS)
    – C. Arnold
    Aug 2, 2022 at 12:12
  • who said that it's 400% max? It depends on the cluster size and file size. For example if the partition uses 2MB cluster and the file all are 1-byte long then it'll increase 209715200% in size
    – phuclv
    Aug 2, 2022 at 13:29

1 Answer 1


Perhaps the OS, Windows, is trying to open archives, such as ISO or ZIP files, to transfer contents item-by-item to the USB flash drive. If that is the case, do not open the archive and drag files separately to the other drive, but drag the whole archive, instead.

You might also try the much more robust robocopy utility built into Windows. robocopy can be faster and more reliable than drag-and-drop in Explorer or using the copy command.

BTW, before doing anything else, use diskmgmt.msc or a third-party tool, such as free DiskGenius to check that the USB flash drive is correctly partitioned for your use. Likely, it should have one partition of ~1 TB (manufacturers do tend to over-state the actual size, but even 300 GB would be sufficient). If the drive has been formatted with multiple partitions, remove all partitions, then create a single partition for all free space, and then format it exFAT, to allow ISO files greater than 4 GB to be copied.

  • Agree with the rest, but I wouldn't use ExFAT. It's far too flimsy to be reliable for a backup. Use something [at least approximating] journaled, which would be NTFS for Windows, preferably on GPT/GUID not MBR.
    – Tetsujin
    Jul 31, 2022 at 17:26
  • @Tetsujin, it depends: for flash, exFAT is perhaps less stressful for writing, but an external HDD, I'd use NTFS. See also superuser.com/questions/257646/… Aug 1, 2022 at 0:58

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