I was always taught that one of the primary uses of a comma is for when you're divvying up unnecessary parts of sentences (I'll just go ahead and assume that was right). So, when I'm constructing sentences I will read the sentence without the sections that have been sectioned off by commas and, if it makes sense, I will proceed on to the next sentence.
One thing I have noticed, though, (particularly with the word "but") is that people tend to construct certain sentences like this:
"I'm not particularly one for this type of music, but I love this song."
Now, in this case, the first part of the sentence is the unnecessary part. So, without the first part, it would read:
"But I love this song."
This obvious makes no sense, as it is out of context and was not a reply to anything. Were it me, I would have constructed it like this:
"I'm not particularly one for this type of music but, I love this song."
This way, without the first part it would read
"I love this song."
which, obviously, makes more sense.
So, which way of constructing the sentence is "right"?