A million gigabytes is correct, though the exact answer would vary depending on whether you asked a hard drive manufacturer (who would say a megabyte is 1,000,000 bytes and thus a petabyte would be 1,000,000 gigabytes) or an operating system developer (who would say a PB is 1,048,576GB seeing as software calculates a MB as 1,048,576 bytes).
When working with the latter system, you can easily convert back and forth between smaller and larger units of storage by multiplying or dividing by 1,024.
For example, if you wanted to convert 888,888,888,888 bytes to gigabytes, you would divide by 1,024 (to first convert to KB), and then divide by 1,024 two more times to get the gigabyte result (827.84GB).
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