Nowhere at all, in any tool
Not seen by the operating system or its disk utilities. Shows a problem with the bridge board, cable, the power path, or a completely dead drive. These are where the four checks above prove useful.
Hardware
There are four things between your files and what is on the monitor — cable, port, power supply and the bridge board within the enclosure — and three of those items are less expensive to eliminate as problems than the drive. Check these things going out from the drive before assuming the worst about your data.
If a hard drive is spinning quietly but never shows up on your computer, that points at the electronics. If a drive makes noises such as clicking, ticking, or if it spins up but will not stay running, that points at the mechanism. Two very different ways to handle each situation.
There are four components between your files and the display on your computer. Three of those components can often be eliminated at a lower cost than the fourth.
A USB cable can fail inside but appear perfectly fine from the outside. It is generally the lowest cost item provided in a complete package. Before you do anything else; swap out the original cable for one that has been tested good.
Do not go through a hub, keyboard or monitor; go directly into the machine. A 2.5-inch bus-powered unit will behave just as if it were a "dead" hard disk when placed in a hub that can't deliver enough amperage to make the drive spin.
3.5-inch externals use bricks outside the case and these are more prone to failure than the drives they contain. Verify voltage, amperage and polarity of your replacement part vs what the original said — a substitute of the correct rating costs a few pounds.
This allows for isolation of whether the issue lies with either the drive itself or with some other component such as your driver or OS in approximately one minute and informs you of which problem exists.
A quiet spinning motion with no appearance suggests the bridge circuit board has failed while having a good internal disk. Clicking or scraping sounds suggest the mechanism of the drive has failed — unplug it and do not try to plug it back in to test.
The same drive may show up missing in one spot, yet show up in another. Each shows a different result.
Not seen by the operating system or its disk utilities. Shows a problem with the bridge board, cable, the power path, or a completely dead drive. These are where the four checks above prove useful.
Drive is recognised and able to communicate. Drive has no assigned drive letter, no recognisable partition, or a file system unknown to Windows. Much better than a physical fault.
Recognised and with an identified partition and an unreadable file system. Do not accept the offered format; formatting will write a new file system on top of what a data recovery would have restored from.
The capacity is correct, so your drive is working; you just lost the partition table (where volumes are located). Everything else remains intact. A few hundred bytes lost.
This is one of the most destructive things people will do to an undetected external. It is being done with good intentions but it causes serious problems.
Most modern WD My Passport and My Book units use hardware-based encryption on their data whether or not they had a user password. The encryption keys reside on the small USB bridge board within the housing, not on the physical media.
Removing the drive and attaching it directly (a perfectly sensible instinct, and one that works fine on a lot of other brands) gives you a complete disk filled with ciphertext. Windows identifies it as unformatted and asks if you want to initialise it. If you allow it to do so then you have done real damage to a drive that previously worked properly. Now the original bridge board is all that exists that can unlock what is on it.
If you've already pulled the drive from the WD enclosure: keep the enclosure and the board. Do not discard either, and send them in with the disk. The recovery process is much more difficult without these items, and sometimes impossible.
Cables are where persistence can help. None of these issues are helped by persistence.
Any noise it did not make before. The mechanical noises such as clicking, grinding, scraping or buzzing while there's no spin. Mechanical failures always worsen over multiple run attempts. The length of time between the first sign of failure & the time the user turns the machine off will greatly affect how much data is recoverable.
It appears, then vanishes, repeatedly. A device that mounts briefly then goes offline is typically a bad bridge or a device stalled on unreadable sectors. Every time a device spins up; there's an increased chance of writing "bad" information to the device.
Windows offers to format, initialise or repair it. There are no recoveries here. All write data. If you have got as far as being offered a fix, your next action should be to decline it and disconnect.
Try a new cable. Then try another direct port right on the box instead of via a hub. Then check the power supply on a desktop unit. Finally, use a completely different PC. These four items can be done at no charge and solve a large share of undetected externals.
You've received confirmation that the device is present and talking. This means it has no drive letter assigned, it has no readable partitions or file systems in format that Windows can read — this is a logical fault rather than a hardware failure.
Not on a WD unit. Most My Passport and My Book units are now encrypted via hardware using a key stored in the USB bridge board. The result is that a bare disk appears as an unformatted disk even though the disk itself may have all its files intact. Ship the enclosure along with the drive.
No. That error message indicates an inability to access/ read the partition or file system — not that your information has disappeared. When formatting is applied, it will write a new structure on top of a structure that a recovery would need to recreate to build back your files.
Almost certainly not. Correct capacity means the drive itself is responding. The lost item is the partition table — a few hundred bytes located at the beginning of the drive that describes where different volume areas start. All other components remain unchanged.
When you hear a new noise that the hard drive has never produced before, each time it attaches to your computer then immediately dismounts, or when Windows offers to either format or repair the drive, at that point you should stop trying and get help, because further attempts reduce what can be recovered.