If, for example, I will disconnect disk that got sda1 and reboot the router the remaining disk will get sda1. There was a situation when I didn't disconnected any disks, but after reboot they've got sdb1 and sdc1 in their share names (and in their block device names). In Linux block device names are not promised to be persistent, especially not for hot-add devices like USB drives.
Please check the link I supplied in previous note, they explain how to make those share names to be persistent between reboots. For example: if I could make that router to assign the share name according to volume label then my problem will be resolved..