On Raspbian there is a service running called "udisks2.service" which handles "auto-mount" of block storage devices (MMC/USB). This service will actually mount the "inactive" part as it simply mounts all the block devices it can find which are not in "/etc/fstab".
And Mender will happily write an update to the inactive part, even though it is mounted. This results in "inconstant" and somewhat undefined behavior, and the update is easily corrupted.
Without having the information that I provided here it is really hard to debug and will only show up as "updates fail" without any specific reason.
- Detect if inactive part is mounted and error out when performing an update