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Mac file names Windows does not allow

A Mac is relaxed about file names. Windows is not. When you copy files from a Mac drive to a PC, some names simply cannot exist on Windows, and a copy tool has to do something with them.

What Windows does not allow

From Microsoft's file naming rules (checked October 30, 2026):

  • The characters < > : " / | ? * are reserved.
  • Reserved names: CON, PRN, AUX, NUL, COM1 to COM9, LPT1 to LPT9, also with an extension such as NUL.txt.
  • "Do not end a file or directory name with a space or a period."
  • Windows folders are normally case-insensitive, so Photo.jpg and photo.jpg count as the same name.

On a Mac, names like Really?.txt, 10:30 call.m4a, notes. or aux.txt are all possible. On a case-sensitive Mac volume, Photo.jpg and photo.jpg can sit in the same folder. Case-sensitive APFS

What Diskferry does with them

  • Each forbidden character is replaced by an underscore: Really?.txt becomes Really_.txt.
  • Trailing dots and spaces are removed, and a reserved name gets an underscore in front: aux.txt becomes _aux.txt.
  • If a name is already taken in the destination folder, for example by its twin with different capitals, the file is saved as photo (2).jpg.
  • The copy report lists every renamed file with its original path and the name it was saved as.
  • The copy is still verified: the renamed file is compared with the original on the Mac drive by SHA-256.
  • Aliases and symbolic links are not copied as files; the report lists each one with the path it points to.
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Other things that look different on Windows

  • Packages such as a Photos library (.photoslibrary) or an app (.app) look like single items in Finder but are folders. Copy the whole folder.
  • Long paths. Microsoft notes that before Windows 10 version 1607 a path was limited to 260 characters, and later versions need a setting to remove the limit. Copy deep folders to a short destination such as D:\Mac.

Questions

Can I rename the files back on a Mac?

Yes. The report lists each original name next to the safe name it was saved under.

Last updated October 30, 2026. Still stuck? Support replies in person.

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