If you still have a warranty, this does not apply to you! Submit a support ticket with WD instead of trying this!
You will also be trying all this at your own risk! No one except yourself will be responsible if you turn your MPWP (My Passport Wireless Pro) into a paper weight!
WARNING: I tried replacing the hard drive with a Samsung EVO 840 SSD on 2017-08-20 and the firmware restore script had issues and it just looped. DO NOT TRY THIS unless you connect to the UART and have an understanding on how arm works and/or hardware hacking.
Now that is out of the way... I'm planning on messing around with the official firmware and extending it. I was thinking about doing Gentoo instead but decided not to compile since there was that battery thing with the first gen that seems to be fixed with a firmware patch and I rather not deal with all that... Anyway, before playing around I wanted to recover on a different hard drive, that way I know if stuff was in internal memory or if it's on the hard drive.
What I've learned so far that's related to recovering or replacing the hard drive:
1. The serial console to see what's going on/troubleshoot is really easy to attach without soldering. The standard 3v3 - 115200 settings worked for me on the SOC connection.
2. The OS is on the hard drive.
3. The initial wifi password is tied to the last 8 characters of hard drive serial number. So if you're replacing the hard drive with a different one, please write it down or take a picture with your phone (what I did).
I did this on a 320GB HD that I'm planning on using for awhile before I eventually switching back to the 2T drive so I know you can go smaller, doubt there's restrictions on going larger...
I thought I would share this info in case anyone else wants to play around or they're unlucky and their hard drive die (please refer to first line of my post).
Step 1 - Download the firmware from WD https://support.wdc.com/downloads.aspx?g=235&lang=en
Step 2 - Format a SD card with the FAT-32 Filesystem
Step 3 - Create an folder on the SD card with the name 'update' (without the '')
Step 4 - Copy the firmware file into the update folder
Step 5 - If you're switching hard drives, this is the time to do it. I will warn you once again, make a note of the last 8 characters of the hard drive's serial number. This will be your WiFi password.
Step 6 - Insert the SD card into your MPWP
Step 7 - Turn on your MPWP and quickly hold the SD/copy button for about 5 seconds. This will trigger the bootloader to restore the firmward that's on the sd card.
Step 8 - Have your favorite beverage while the hard drive led flashes, that means it's doing it's thing...
Step 9 - The device will reboot once it's done and then you're good to setup
Edit - Forgot to mention, part of the setup process is formatting your storage area with exfat.