Thanks guys.
I think this is probably the case, because shortly after I was able to upload the same file with no issue. In the process of backing up, I found an unexpected EOF error on a macro, so it seems like the SD was flaky.
Using (the awesome) RFM I was able to pull almost every file off the card (even though windows wouldn't properly mount it), and push them all to a new card. Boom, up and running without missing a beat.
Oddly, despite the DWC working on my local computer, when I try to load it from an external network, I get some errors in the console:
b5b457dd-760d-4f3b-b4d4-e5369b3eceba-image.png
It seems odd, since I don't know why it would be using different files. But, I'll reinstall DWC and see if that fixes it.
Fixed after reinstall