There were 12.6 - 4.6 = <<12.6-4.6=8>>8 gigabytes used after deleting some files.
Then, the used space went up to 8 + 2 = <<8+2=10>>10 gigabytes after storing other files.
Thus, the new 20 gigabytes external drive will have 20 - 10 = <<20-10=10>>10 gigabytes free.
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