Thanks for your message.
Unfortunately, WESTERN DIGITAL WD10EADS - 00M2B0 is not on hard drive Synology support list.
According our development team check your system, we found some error sector on harddisk caused degraded mode.The system unmount the second harddisk.
Jun 7 10:12:36 kernel: end_request: I/O error, dev sdb, sector 9430155
Jun 7 10:12:36 kernel: md: super_written gets error=-5, uptodate=0
Jun 7 10:12:36 kernel: raid1: Disk failure on sdb3, disabling device.
Jun 7 10:12:36 kernel: ^IOperation continuing on 1 devices
We suggest you to backup as the first priority.After recovering your data, please use disk diagnostic tools and perform a SMART test on your hard disk checking to see if your hard disk is healthy or not. .
[WD HDD: Lifeguard]
http://support.wdc.com/product/download.asp?level1=6
If your hard disk is ok, or if you need to change to a new hard disk, insert the (new) hard disk into the system and re-install the latest firmware again.
If you have further questions or suggestions, please feel free to contact Synology support again.
Best Regards,
Hugo Chen