USB drive not being read

After a recent upgrade (to 7.19rc1) I saw my container was not running and saw that the reason was that my USB drive’s files were no longer visible - the disk was recognized but with indication file system unknown (it is an ext4, no partition table).
I downgraded (to 7.18.2) and I once again saw the files on the USB, although my container would still not start. So I rebooted again and now the file system on the USB is once again not seen by the system.
(I attached the USB drive to a linux box and could read the disk without problems.)
Further reboots have no effect.
Any suggestions, besides reformatting and starting over with my container? What could the issue be?

Hi, i think it is a bug. Same here: http://forum.mikrotik.com/t/rb3011uias-v7-usb-pendrive-not-mounting-after-reboot/181926/1
But nobody answer…!