Yes: something is still using the drive
I don't mean to be flip, but the first error message you quoted gave the reason. The interesting question is what is still using the drive. Likely it's related to you opening it in the Backup&Restore app, but that's pure guesswork on my part.
I wouldn't sweat the 'removal' issue at this point, since you can always force it by shutting down the system. The more important question to my way of thinking is, do you have a usable backup image?