My Book on QNAP TS-251 stopped working (too many disk attached?)

I recently bought NAS QNAP TS-251 and I connected USB hub to connect other external drives.
So far I had 4 external drives pluged in and everything was OK, until I connected disk number 5. Not to USB hub, but directly to NAS. From this moment my external drive WD My Book stopped working and I’ve got this warning below. After I unplugged the 5th drive and reboot NAS, My Book show up again.

Server Name: …
IP Address:
Date/Time: 28.02.2017 10:17:55
Level: Warning
WD My Book 1110 Read I/O error, sense_key=0x0, asc=0x0, ascq=0x0, CDB=28 00 35 7e d5 c0 00 00 08 00 .

Is it possible to diagnose what happens? Is it possible that there is a problem with disc or I just reached maximum number of attached drives? Why only this disc stopped working? I have others (another WD and 2x Samsung)

Thank you


Have you tried to connecting the drive directly to a computer to determine if the I/O error is caused by the drive itself or the connection to your QNAP

yes on my PC (Win10) everything is OK. No errors.