Apple has sent out letters to its developers testing the iOS 8 Beta and OS X 10.10 Yosemite informing them that it will be wiping all CloudKit-based data stores on July 7th. The features which will get affected are iCloud Photo Library, iCloud Drive ad Mail Drop.
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
Photos and videos will get automatically restored, as they are not getting deleted from the local storage. But the affected attachments sent through Mail Drop and iCloud Drive will not transfer across automatically on the release of the next Beta seed and would require user to resend the messages. After the data wipe, most probably the new seeds of iOS 8 and OS X Yosemite will follow.
(via 9To5 Mac)