When an update to Windows is installed, if that update is a GDR (general distribution release) update, the possibility exists that a hotfix exists for the same files but in an earlier version. This hotfix would have been developed separatly within Microsoft for the specific customer problem that the hotfix fixes, but as it is not part of the service pack/security update cycle it does not have an obvious version number, and so could easily be installed out of sequence. Therefore the, the hotfix files for that GDR are copied to the %windir%\$hf_mig$ folder. This allows migration to the appropriate files if you later install a hotfix or service pack that includes earlier versions of these files. For example, consider the following scenario:
- You apply a security update that installs a GDR version of File.dll with a version number of 5.2.3790.1000 and copies a hotfix version of File.dll with a version number of 5.2.3790.1000 to the %windir%\$hf_mig$ folder.
- You apply a hotfix that includes a hotfix version of File.dll with a version number of 5.2.3790.0000.
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