Cannot be opened for writing

Hi, I have over 10000 mp3's on a seagate portable usb hardrive. Many of the mp3's have tags that I want to tag. When I try to tag a song I get the message the Mp3 cannot be opened for writing. Any help would be great!!

What other programs do you run at the same time?
Esp. WMP likes to grab newly attached drives and opens files as it tries to "synchronize" them.
Also: have you finished the "autorun" procedure where Windows asks what to do with the contents on a drive? Or ist that dialog still open?
Then, if you have Vista or Windows 7 and use the slideshow sidebar application then that may try to look up media on that drive.
Check the task manager if there is an unwanted slide show process or WMP you have not started and kill these.
FInally: if that collection comes "from a friend" check the integrity of the mp3s first e.g. with foobar or mp3val. It these utilities show dubious files then repair them first before editing the tags.