I would say the number varies.
Answer:
Bats cannot run so it would be almost impossible for them to take off from the ground. A major advantage to hanging upside down is that bats do not need to generate lift to begin flight. They just drop out of their bed, open their wings and off they go.
Explanation:
In case the first drive fails, the data will all be backed up onto the second drive, a copy will still be intact.
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Hope this helped!~