I am working on acquiring USB>USB cable to connect the WD hard drive to the Airport extreme and hopefully i will see the WD hard drive in the Finder window so i can format it.
Sorry but in the previous discussion the WD drive was in a Synology NAS.
I am trying to hook up a Synology disk station with a western digital 4T hard drive in it. I think i need to format the WD hard drive, but I can't find it in Finder.
The format of the drive is purely a NAS consideration.. The procedure to make the NAS accessible by your Mac and Time Machine is in the Synology instructions.. you need to read them and carry them out.
https://www.synology.com/en-us/knowledgebase/DSM/tutorial/Backup/How_to_back_up_files_from_Mac_to_Synology_NAS_with_Time_Machine
You can plug the WD drive in a USB enclosure into the Mac to format it HFS+ which is the only format the Airport can use for backups. But it will be super slow.. and nor can you put the WD disk back in the Synology as that will not work. Synology must format the disk in the correct way for its internal OS.