why won't my devices recognize my my book external hard drive for iMac?

It connects by USB? Run Disk Utility. Does it appear in the Disk Utility sidebar?

If it does, and this drive was sold as a "Windows" external drive, you should reformat (Erase) it. Select the DRIVE, not the volume indented under the drive, in the Disk Utility sidebar. Go to the Erase tab.

Set the Format to Mac OS Extended (Journaled). Name it whatever you want. Click Erase. It should take less than one minute to complete. It everything is working properly, it should appear in Finder windows under DEVICES.

You go to Applications - Utilities - Terminal, then click your Hard Disk from Finder and assuming you have no data, Command-C the disk and in Terminal, type diskutil partitionDisk __ 1 [GPT]Format part1[GPT] part1(The name you want) part1(A little bit less than your hard disk cap.)

(__ is when you do Command-V)

Example:

1TB My Book

And you want to name it “My Disk”

diskutil partitionDisk (Command-V) 1 [GPT]Format part1[GPT] part1MyDisk part199G

The G at the end is GB.

If it is larger, you can use

T - TB

B - Bytes

OR just put 90%.

If you want to find out more, type man man

And it connects by USB? I have a WD external drive with both USB and FireWire. Which port on your Mac does it connect to...?

You connect the drive and you connect the power supply it comes with; there should some kind of button (or maybe a switch) to turn it ON. When you power it ON, do you hear the hard drive "spin up" along with lights on the front of case?

Open Finder > Preferences check what you want to show, where you want.😉

Try another USB cable, or try another hard drive (if you have one) using that cable. Basically confirm it's not a bad cable (or that he USB port on the Mac that it's connected to is not malfunctioning).

After that... Since said it doesn't even appear in Disk Utility, I think you got a bad external drive, especially since it has never worked. I would contact WD about getting a replacement.