OK, maybe that's it. I am wondering, since I can't go back and burn anything else to my DVD, would it have been possible to burn several things at once? Multiburn? on that DVD-R type of disc?
Can I remove or erase what I have on there and try again, only with two items this time?
Yup... That's it! I just learned something new, myself...that Finder doesn't offer the same burn options as do some/all(?) other burn utilities.
The + and - disc attributes refer to their compatibility with different types of optical drive.
Early on, there were two competing formats that were supported by various entities, and a + disc wouldn't work with a - drive, and vice versa, but today, many/most/all(?) drives, yours included, support both formats.
So... Yes, you could have burned any number of files and folders, up to your disc's capacity, to the DVD you bought.
And no, you can't erase your DVD; only a DVD with the "W" attribute can be erased and re-burned.