RAID6 Array needs to be manually reassembled after every reboot

Hi,

I'm Running OMV 3.0.99 on a Odroid XU4 with 4 x 2TB in RAID6 through a USB3 interface (JMicron). Every time I reboot I get an email reporting a degraded array and the array fails to mount automatically. However If I do the following the array mounts without problems:
mdadm --assemble /dev/md0 /dev/sd[abcd] --verbose --force

Sometimes after doing this one of the drives will be missing and I have to add it manually:
mdadm /dev/md0 --add /dev/sda

The array is reported to be clean and runs normally after doing this. No SMART errors either.

Any ideas?

Thanks!