Presumably I'm not sending the originals. Also, powered down, hard drives are pretty robust - their read/write heads are locked up. Remember - those drives have been bouncing around a LOT to get to you through retail/wholesale channels.
[edit] Also - they obviously had to buy hard drives for the destination, right? Presumably, early stage google was all about the open-chassis on plywood slam drives in approach, and wouldn't have been averse to loading up 30 or 40 drives, mailing them out, and simply diffing and resyncing as needed if 1 or 2 drives had issues, which would have been unlikely.
Also, presuming you needed to have one of your people go to the remote data center, there is always the old, move the datacenter with hard drives in my luggage trick. Even in 2000 you could put a lot of data into carry-on.
You could make up any small number of botched drives with over-the-wire transfers. If it were a sufficient number of drives, you could simply re-image and re-transmit the data.
When they're adequately backed up, as in this circumstance, why not? Send the data in triplicate if you want, just to avoid the hassle of re-sending any that get damaged. Regardless, the cost would be trivial, even for young-Google.
Edit: I'm not implying that USPS is necessarily unreliable, I'm curious to hear if this is common practice.