Discourse markers are signposts. They do not all punctuate the same way.
Which way it turns
The file is gone. However, I have a backup. The file is gone; however, I have a backup.
Not a joiner
However is not and / but. A comma alone before however is a splice.
The file is gone, however I have a backup. — no
The file is gone. However, I have a backup. — yes