_unknown User and Lack of Permission

It's been about a month or so since I last attached my external drive. Never had any problems.

Today, I attached it because there were two things I wanted to accomplish:

1) Copy a file from an old Time Machine backup (of a previous installation).
2) Edit the tags and information of some of the audio in my iTunes library (I store it all on there).

I tried iTunes first. I couldn't edit the information (Artist, Year, etc.) for the majority of my files. I navigated to the their directory in Finder. Pressed cmd+i and noticed a user called "_unknown" (with RW privleges. Alongside staff and everyone which are read only). I wasn't listed (whereas I was for those few files I could still edit in iTunes, with Read & Write).

So, then I tried copying that file I was after instead. I couldn't even reach the file. Just trying to enter the back up copy of the Documents folder brought up:

===
The folder "Documents" can't be opened because you don't have permission to see its contents
===

cmd+i again and noticed the same thing. I'm not listed but "_unknown" is. Again, _unknown has RW privileges. The only other user is "everyone" with no access.

I manually added myself back onto that Documents folder and copied the file I was looking for. I really don't want to do that for every other file that's experiencing a problem. Is there an automated way of doing this?

Through the Terminal I'm assuming.

Much thanks!