What Would Prevent File Transfer To an External Drive?

I just bought an external HD, connected it to my Mac and transferred files to it. Then I connected it to my TC so I could access it over the network. Today, I attempted to transfer a file to it over wifi, and it would work. I can pull files from the drive, but can’t transfer to it. I’ve tried drag-and-drop and copy-and-paste. In either case, the icon appears on the external HD in the Finder window and the progress meter shows my transfer in progress. However, once the transfer if complete, the file disappears. I’ve even used search, but it’s just not there. I can transfer from my Mac to my TC, but I can’t transfer from my Mac to the external connected to the TC. Any ideas?
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Cameron