non-erasable files with special characters

Your only real option is to back up some of the key files, reformat the drive (using a Windows machine) and then restore the backed up files.

There should be several threads on how to do this. If you cannot find one then let me know and I will write some instructions up.

It might be worth trying to delete the files by opening a command prompt and using dos commands to go to the desired directory(folder), then use del *.* to delete everything in that directory.
Double check that you are in the correct directory, otherwise you might delete more than expected.

If you go the command prompt route, the command above will only get rid of files, it won't get rid of the folder that contains special characters. So if you want to try the command prompt route, you'd probably get better results by using the following command rd /s e:\Garmin\NewFiles The "rd" command stands for Remove Directory. The "/s" indicates to remove the specified directory and all files and directories that it contains. If this command works (i.e. the special characters don't cause it to fail) you'll then have to use the following command to recreate an empty new files directory md e:\Garmin\NewFiles

One other tip is that you may need to run the command prompt "as administrator" so that it has the appropriate authority to remove directories and files.