I ran into this issue after a Windows 7 reinstall and attempts to copy old Musicbee library and/or settings folder. My music file folder was left untouched on a separate drive when I reinstalled Windows 7.
Here is the solution:
"Access denied" issues are caused by the Windows folder/file security permissions for your music folder/files. Files in a folder are normally set to inherit permissions from the parent folder, so you would likely want to change the top parent folder permissions, which will propagate to sub folders and files.
1) Add your Windows user account :
Folder > right click > properties > Security tab
If your Windows user is not in the list 'Group or user names', you need to add it :
Edit > add >
enter your user name in the box, click 'Check Names'
your user name should appear, select, click OK.
2) Take ownership of the folder :
Folder > right click > properties > Security tab > Advanced > Owner tab > Current owner
If Current owner is not your Windows user account, you want to change this setting. Click Edit and select your user.
Click OK, OK.
You can now also remove the user that previously had ownership, if that is a user name of a long number-string, which indicates it may be residue of previous windows installation that owned this folder.
Click OK.
3) Set permissions :
Folder > right click > properties > security tab > select your user > Permissions for ... > Full control, Allow [v]
OK, OK.
You should now have access to edit the files as expected, and Musicbee will also have access without need to run it as administrator.