These are important diagnostic informations for WD linux developers about my case:
I have found that MyCloud problem mounting external HDD are specific to USB3.0 devices.
I have done the following:
Connected the external HDD to rear port before booting MyCloud NAS (firmware 4.4.xxx)
Acquired SSH root prompt on MyCloud device
Executed this command:
dmesg | grep usb
I got the following log:
[ 4.241051] calling usb_init+0x0/0x178 @ 1
[ 4.241430] usbcore: registered new interface driver usbfs
[ 4.241776] usbcore: registered new interface driver hub
[ 4.242013] usbcore: registered new device driver usb
[ 4.242042] initcall usb_init+0x0/0x178 returned 0 after 0 usecs
[ 8.346294] /files/sq_kernel/linux/drivers/usb/core/inode.c: creating file 'devices'
[ 8.354093] /files/sq_kernel/linux/drivers/usb/core/inode.c: creating file '001'
[ 8.375139] usb usb1: default language 0x0409
[ 8.379544] usb usb1: udev 1, busnum 1, minor = 0
[ 8.384277] usb usb1: New USB device found, idVendor=1d6b, idProduct=0002
[ 8.399601] usb usb1: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 8.406873] usb usb1: Product: xHCI Host Controller
[ 8.411777] usb usb1: Manufacturer: Linux 3.2.26 xhci-hcd
[ 8.417214] usb usb1: SerialNumber: xhci-hcd
[ 8.421951] usb usb1: usb_probe_device
[ 8.425752] usb usb1: configuration #1 chosen from 1 choice
[ 8.441997] usb usb1: adding 1-0:1.0 (config #1, interface 0)
[ 8.448072] hub 1-0:1.0: usb_probe_interface
[ 8.452371] hub 1-0:1.0: usb_probe_interface - got id
[ 8.469079] hub 1-0:1.0: no power switching (usb 1.0)
[ 8.507707] /files/sq_kernel/linux/drivers/usb/core/inode.c: creating file '001'
[ 8.520190] /files/sq_kernel/linux/drivers/usb/core/inode.c: creating file '002'
[ 8.534807] usb usb2: skipped 1 descriptor after endpoint
[ 8.540262] usb usb2: default language 0x0409
[ 8.544664] usb usb2: udev 1, busnum 2, minor = 128
[ 8.549582] usb usb2: New USB device found, idVendor=1d6b, idProduct=0003
[ 8.556419] usb usb2: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 8.563680] usb usb2: Product: xHCI Host Controller
[ 8.568598] usb usb2: Manufacturer: Linux 3.2.26 xhci-hcd
[ 8.574026] usb usb2: SerialNumber: xhci-hcd
[ 8.578737] usb usb2: usb_probe_device
[ 8.582516] usb usb2: configuration #1 chosen from 1 choice
[ 8.618584] usb usb2: adding 2-0:1.0 (config #1, interface 0)
[ 8.624757] hub 2-0:1.0: usb_probe_interface
[ 8.624768] hub 2-0:1.0: usb_probe_interface - got id
[ 8.624816] hub 2-0:1.0: no power switching (usb 1.0)
[ 8.625238] /files/sq_kernel/linux/drivers/usb/core/inode.c: creating file '001'
[ 8.625583] calling usb_stor_init+0x0/0x44 @ 1
[ 8.625900] usbcore: registered new interface driver usb-storage
[ 8.625934] initcall usb_stor_init+0x0/0x44 returned 0 after 318 usecs
[ 9.839098] usb 2-1: new SuperSpeed USB device number 2 using xhci-hcd
[ 9.889558] usb 2-1: skipped 1 descriptor after endpoint
[ 9.889571] usb 2-1: skipped 1 descriptor after endpoint
[ 9.904478] usb 2-1: default language 0x0409
[ 9.925339] usb 2-1: udev 2, busnum 2, minor = 129
[ 9.925351] usb 2-1: New USB device found, idVendor=0dd8, idProduct=ac00
[ 9.925364] usb 2-1: New USB device strings: Mfr=4, Product=5, SerialNumber=6
[ 9.925374] usb 2-1: Product: USB to S-ATA
[ 9.925382] usb 2-1: Manufacturer: Genesyslogic
[ 9.925389] usb 2-1: SerialNumber: 0000000000009104
[ 10.175866] usb 2-1: usb_probe_device
[ 10.179571] usb 2-1: configuration #1 chosen from 1 choice
[ 10.261053] usb 2-1: Successful Endpoint Configure command
[ 10.280946] usb 2-1: adding 2-1:1.0 (config #1, interface 0)
[ 10.485157] usb-storage 2-1:1.0: usb_probe_interface
[ 10.485181] usb-storage 2-1:1.0: usb_probe_interface - got id
[ 10.505319] scsi2 : usb-storage 2-1:1.0
[ 10.506228] /files/sq_kernel/linux/drivers/usb/core/inode.c: creating file '002'
[ 141.904958] usb 2-1: device not accepting address 2, error -62
[ 192.032328] usb 2-1: Cannot reset HCD device state
[ 242.162174] usb 2-1: Cannot reset HCD device state
[ 292.293020] usb 2-1: Cannot reset HCD device state
[ 292.378780] usb 2-1: USB disconnect, device number 2
[ 292.383791] usb 2-1: unregistering device
[ 292.387880] usb 2-1: unregistering interface 2-1:1.0
[ 292.418909] usb 2-1: usb_disable_device nuking all URBs
[ 342.433144] hub 2-0:1.0: couldn't allocate port 1 usb_device
[ 141.904958] usb 2-1: device not accepting address 2, error -62
This indicates there are some problem initializing USB3.0 devices properly !!!
The latest firmware is somewhat flawed and should be patched, it is highly probable
that something is wrong with xhci_hcd driver.
Hope this helps,
Paolo Marani