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I have already set up multiple time File History without problem but this time everything seems fine but when the "backup" is done, FileHistory\xx\xx\Data\C\Users\xx is empty.

In Event Viewer, under Application and Services Logs, Microsoft,Windows, FileHistory-Engine, FileHistory backup log, I have many errors like this "File was not backed up due to its full path exceeding MAX_PATH limit or containing unsupported characters". This somehow prevent the backup.

I have removed the files with a too long path and File History is now working as expected.

I thought that the files with a too long path were backup in xx\xx\Data\$OF. So why did my files prevented File History from working?

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First of all, long path name file will be backed up in $OF folder with renamed name instead of its original name.

According to Microsoft official document:

Starting in Windows 10, version 1607, MAX_PATH limitations have been removed from common Win32 file and directory functions. However, you must opt-in to the new behavior.

A registry key allows you to enable or disable the new long path behavior.

To enable long path behavior set the registry key at HKLM\SYSTEM\CurrentControlSet\Control\FileSystem LongPathsEnabled (Type: REG_DWORD).

The key's value will be cached by the system (per process) after the first call to an affected Win32 file or directory function (list follows). The registry key will not be reloaded during the lifetime of the process. In order for all apps on the system to recognize the value of the key, a reboot might be required because some processes may have started before the key was set.

The registry key can also be controlled via Group Policy at

Computer Configuration > Administrative Templates > System > Filesystem > Enable win32 long paths.

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