Here is the sentence again:

The computer began to wail like an ambulance, startling poor Robert, who only wanted to retrieve his paper, unfortunately, a virus had immobilized the hard drive.

The computer began to wail like an ambulance is a main clause. A virus had immobilized the hard drive is another main clause. These two main clauses have been joined with only a comma, thus creating a comma splice, a major error in sentence structure.

To fix the sentence above, you could use a semicolon between paper and unfortunately.

Go to the next sentence.

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