Locked out of my external hard drive

I went into my MacBook Pro this morning and found that I cannot get into the external drive that contains my working files. It has a small lock on it, and when I go to Get Info, it tells me that every account ("me", "admin" and "everyone") has "Custom" permissions to get into the drive, and won't allow me to alter those permissions from the Get Info dialog window.

Next, I went to reboot my system to see if that would help, but all I got was the Apple symbol on startup and a spinning wheel below it. After waiting about 15 minutes I reached for my trusty bootable backup disk and fired up the system again. I'm currently running from my bootable backup ... slowly ... but it's two months old and I'd prefer not to clone it to my internal hard drive if I don't have to do it.

I may have done all this myself, but I don't know what I did if I did. What can I do to unlock my external drive and gain access to all my working files again? And if there's some kind of boneheaded move that I made, can you suggest what it was so I don't make the same mistake again? Any help would be greatly appreciated.