> I wonder how this process worked in practice. Do you simply send your only one set of hard disks and hope for the best?

No, you don't put your own disks in the Snowmobile/Snowball/Snowclone. It contains disks, so when it arrives you connect it to your network and copy data onto it, and then it is driven to an Amazon datacentre.

See, e.g. https://docs.aws.amazon.com/snowball/latest/developer-guide/...

https://aws.amazon.com/blogs/aws/aws-importexport-snowball-t...