I found a Western Digital 8 TB My Book external USB disk drive with part number WDBBGB0080HBK-NA in a box with a number of other disk drives, but the box did not contain the power adapter for that drive. The back of the drive has "12V" next to the place where the power adapter connects to the drive but no indication of the amps needed. You can find a listing of the specs for power adapters needed for various disk drives at Western Digital's Power Adapters for WD Products webpage. That page shows that the My Book drives with the following part numbers use AC adapters that are 12 volts, 1.5 amps, and 18 watts.
Note: you can find an explanation of the format Western Digital uses for their model numbers in the PDF file downloadable from Western Digital at Model Numbers - Branded Products or here. For older drives, Western Digital also provides a Retired Model and Order Number Formats for WD Products document that is also available here.
In this case, the "0080H" indicates the drive is a 3.5" drive with a capacity of 8TB.
I didn't have a power adapter from another Western Digital drive handy that met the required specifications for volts, amps, and watts. I could order a needed adapter online—the WD 18-watt power adapter is model number WDPS037RNN while the 36-watt version is model WDPS039RNN—from Amazon and receive the adapter in just a few days, but I needed to check on whether that particular drive held a needed backup immediately. I had a Seagate Backup Plus 5 TB USB 3.0 Desktop Drive available with an ADS-18E-12N power adapter which lists an output of 12V and 1.5A on the adapter, so I figured I could use that drive, since though that adapter doesn't list the wattage, the formula to calculate watts if the amperage and voltage are known is amps x volts = watts and 12 x 1.5 = 18 (Obtain Watts from Amps and Volts provides an online converter). So I was able to use the Shenzhen Honor Electronic Co. ADS-18E-12N adapter from the Seagate drive on the Western Digital drive. Unfortunately, though it held a backup for the system for which I needed to recover files, that backup did not contain the needed files.