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I noticed that one of my drives was full after moving some files onto it:

enter image description here

I thought it unlikely that copying files of random sizes would result in exactly filling the available space, so I became suspicious.

I then dragged a file that was 1.57 MB off the drive and onto my desktop (via shift+drag). The drive's free space then said 1.52 MB, which didn't make sense. I then clicked "Undo Move" and it would not move the file back, claiming there was not enough free space.

Can anyone make sense of this? There is nothing else being written to the drive. I'm guessing it has something to do with the internal structure of the drive (e.g. fragmentation [although it says 0% fragmented]), and this is a drive on a virtual machine.

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Triynko
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1 Answers1

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There are various possibilities why a file occupies more disk space when copied to disk than its actual size is:

  • NTFS compression on the source disk can reduce the actually needed storage there
  • A large cluster size on the destination disk than on the source disk can lead to a higher occupied space on disk
  • NTFS shadow copies (used by Windows System Restore Points) are implemented using a copy-on-write strategy. This means, you have a full hard disk state of a previous date, but only if data is actually written, the previous state of this data is saved and occupies disk space.
  • The internal structures of NTFS like the Master File Table (MFT) might increase when new files are copied.

Use a disk space management tool to find out which folders occupy most of your disk space and which space they actually occupy on your disk. Make sure to run them "As Administrator" so that these tools are able to see the whole content of your disk.

There are two features of my tool TreeSize Professional (fully functional 30 day trial available) which might help in this situation:

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Joachim Marder
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