My Passport external drive suddenly read-only on MacBook Pro

Hi, I’ve searched the forums and, while I’ve found many threads detailing why a Passport drive would be read-only on Mac by default, I’ve yet to come across a thread explaining what to do when my drive, which has been formatted by my Mac and I’ve been reading and writing to for years, suddenly switches to read-only mode. Is there a way to open this up to read/write without erasing and reformatting? I use this drive as my backup. I DO have a secondary backup, so it wouldn’t be a total tragedy if I did have to reformat, but it would be a pain. Any suggestions?

This is for my Passport 500GB external drive, using a MacBook Pro running OSX 10.6.8.


Does the same happens if you use the drive on a different computer?

Check the link below to see if changing the ownership of the files and folders helps.