SQL Server 2008 R2 .mdfs write protected
Might be OS-level access rights--every now and then I hit something like this. In Windows Explorer, right click on the file and select properties. Go to the Security tab. Are there any entries in the "Group or usernames:" box? These are the entities that can manipulate the file. Yes, even if you are local and domain admin, you can be blocked from accessing a file.

However, you can't be blocked from managing access rights over the file. Click on "Add...", grant yourself (or whoever) "Full Control", and you should be good. (If, of course, this is the actual problem.)