JSI Tip 10445. USMT 2.6.x (User State Migration Tool) has a 259-character limit for a file path in Windows?

When you run USMT version 2.6.x, a drive + folder path + file name that exceeds the 259 characters may result in the user being unable to access the resource.

You should limit your drive + folder path to a maximum of 245 characters, to leave sufficient space for an 8.3 file name, but I suggest a more practical limit of 240 characters for your folder path.

See Naming a File.