From WD:
Please follow the instructions below to recover the WD Sentinel Server image from USB — WD FactoryRestore Tool.
Contact support for the WD Factory Recovery Tool
Download the Recovery tool
Extract the files from zip file to a USB drive formatted with FAT32 file system (must be USB 2.0 and between 8GB nad 32GB)
Insert USB drive into the DS5100/6100 rear USB port
With the power on, hold down the Recovery button to boot from the USB drive
Where is the Recovery Tool Download located?
Thanks
BillF