it may be some sort of driver issue. can you see the other files on your "sd card" when connected to the PC?
you should get a pop up asking what you want to do when you plug the phone in. if you select open to see files,you should then see htc one internal storage. click on it,and it should list the "sd card" contents.
you should be able to just drag and drop files onto this screen(assuming you are on stock 4.1)
The Delivery Guy
If someone has helped you,please click the THANKS button on that post.
Please post questions so others can benefit from the answers.
donate to my device fund