The very fact that there is no explanation of how the storage is done in detail ought to make you worry. It certainly looks as if files are identified by their

The very fact that there is no explanation of how the storage is done in detail ought to make you worry. It certainly looks as if files are identified by their (cryptographic?) checksum across accounts even if those accounts are not sharing anything officially. To do that, and to distribute that data from a single point means that someone other than you have a key to read those data. You will have to protect your data by encryption yourself before uploading. Perhaps you should evaluate Tarsnap from http://www.tarsnap.com/ http://www.tarsnap.com/ which is in public beta as of this writing. It provides a neat backup service and has a publically readable/accessible description of what measures that are taken to protect your data from adversaries. I have not tried it myself, but I happen to know that Colin Percival knows what he is doing.