Hi Annie, thanks for the question. The Google Storage Bucket is the default and is handled by the POCS system assuming the unit is connected to the internet. The external hard drive method would mostly be used in the cases where there is limited or no internet with the idea that someone could collect the hard drive every few months and take it to a place with better internet.
Note that for the Google Storage Bucket to work you need to get an official “key” from us, which is usually something we do once you have the control computer ready to go.
Let me know if you have any more questions about it!