"Only a single DVD movie is at least 4.7 GB, So there should be a lot of data on the Internet."
The usage is correct, but confusing, and the statement is false. It literally says "Only one DVD movie (in the entire world/collection) is 4.7GB or more, so there should be a lot of data on the Internet"
This raises the question of why having only one DVD at 4.7GB leads to the conclusion that there is a lot of data on the Internet.
I suspect your real question is whether you can split a sentence like this:
There are 1000 DVDs available online so one may expect total DVD based Internet data to be low, only a single DVD movie is at least 4.7 GB, so there should be a lot of data on the Internet.
The answer is yes, you can split it, but with a slight word change. Replace "Only" with "However," and you're fine to split. Some older style guides dislike starting a sentence with "however" when used as a connection (like starting with "but"). However, (:P) modern style guides allow it.
Some may argue that "Only" means "But" and "However" means "But", so "However" should be interchangible with "Only", but this is false. Remember that "But" cannot be used at the beggining of a sentence (it is a coordinating conjunction designed to join two (independent) clauses within a sentence together). Similarly, "Only" when used as a subordinating conjunction (not as in "Only one thing has this") can only act on two clauses (independent to dependent) within a sentence.