External Hard Drive became Read Only All of a Sudden

My external hard drive (Mac OS Extended) became read-only all of a sudden. This happened after an unsuccessful Time Machine backup from yesterday due to inadequate disk space. After I cleaned up some space and initiated a Time Machine backup manually, the drive became read-only. "Get info" shows the following. As you can see, under sharing & permissions, it says "You can only read", and there is no way for me to modify the permissions even if I click the lock on the lower right.

I also ran first aid in disk utility and it failed with the following error code:

My 1TB drive is almost full and the last thing I would want to do is erase and reformat, which would require me to buy a new external hard drive just to transfer the data. I looked everywhere online and most are solutions to drives formatted as NTFS, but mine is mac OS extended. And I'm running OS High Sierra. Please help me! Thanks!