Apple has just informed developers that it will be wiping all CloudKit-based data stores on July 7th. This includes iCloud Photo Library, Mail Drop and iCloud Drive. Apple warned developers that this might happen with the release of the first iOS beta and it is not uncommon for wipes to happen several times during iOS and OS X beta cycles.
We will be performing a server-side data wipe on all CloudKit public and private databases for iOS 8 beta and OS X Yosemite Developer Preview users on July 7, 2014. The following iCloud features will be affected: iCloud Drive, iCloud Photo Library, and Mail Drop.
Photos and videos stored in iCloud Photo Library will remain on their original iOS 8 devices and will upload to iCloud Photo Library again automatically. iCloud Drive can be re-enabled from Set up Assistant after upgrade. If you choose to store your documents in iCloud Drive, your Documents & Data will automatically be copied to iCloud Drive. iCloud Drive will not update across earlier seeds or operating systems. Attachments sent through Mail Drop will expire and need to be resent after you upgrade.
If you have any questions, visit the Apple Developer Forums.
Apple Developer Technical Support
Affected files for Mail Drop and iCloud Drive will not transfer across automatically on the release of the next beta seed. Photos and videos will automatically restore, however, as they are not deleted from local storage. It is likely that new seeds of iOS 8 and OS X Yosemite will follow this data wipe, as new seeds of these operating systems are already due.