This article describes that there is a boot failure after upgrading FortiGate to v7.0.15 from any version lower than 7.0.14.
Scope
FortiGate v7.0.15.
Solution
Upgrading firmware from version 6.4.4 (version lower than 7.0.14.) to version 7.0.15 directly will cause the system integrity problem.
The error message ‘failed verification on /data/datafs.tar.gz’ should only occur when upgrading to v7.0.15 from any version lower than v7.0.14 once skipped the upgrade path and upgraded directly to v7.0.15.
Before upgrading to v7.0.15, the system has to be on the base of v7.0.14.
If the device is directly upgrading to v7.0.15 from any version lower than v7.0.14, roll back to the backup image from BIOS to fix the problem.