Several users are reporting that the 4.0.x Linux kernel contains a bug which corrupts users filesystems.
To be precise it’s an EXT4 RAID issue. Any stable kernel released since mid-April probably has the bug. It appears that a “fix” to a problem present since Linux 3.14-rc1 is what’s causing the EXT4 RAID corruption issue on Linux 4.0.x.
This problem doesn’t occur for distributions like Ubuntu, RHEL, and other fixed-release distributions that don’t tend to update major versions of their kernel post-release.
The bug is still unpatched in the latest 4.0.4 release. The issue is addressed now and a patch is available and will be included in stable updates ASAP.