Take another look at the original sentence:
You wanted to fix it this way:
Fixing this fused sentence with but alone is a problem. When you use a coordinating conjunction like but to connect two main clauses, you must also use a comma. In the example above, the comma is missing. In addition, there is already a contrast word, however. Two contrast words for one main clause is unnecessary repetition.
You might want to consult the rules for fixing comma splices and fused sentences.