Hello!
I just upgraded My Cloud (1st gen) from 2TB to 4TB and it worked flawless.
Let me share here. Someone might want do the same process 
Important: If your device is dead or with failure this guide is not for you. In this cases search for “unbrick” or take a look at this folder: GDrive WDMyCloud posted by @Fox_exe
Important Note: It has no guarantees. Do it at your own risk
Ok. What you need before you get started:
- 2 usb sticks (you can use 1 if you reuse it - gonna take more effort)
- working My Cloud
- new bigger hdd
- working PC with 2 SATA ports
- Download Clonezilla: alternative stable (Ubuntu-based)
Select: file type iso- Burn Clonezilla iso to your usb stick. I have used Rufus 3.8
- Use your second usb stick to create a Ubuntu Live USB
- Remove HDD from your My Cloud
- Remove all HDD SATA connections from your PC (to avoid data loss)
- Connect your original hdd and your new bigger hdd to your PC (sata connections)
- Plug your Clonezilla USB Stick and boot your PC.
On Clonezilla Menu Select:
- Clonezilla Live (Default settings)
- Your language and keyboard
- Start_Clonezilla
- device-device work directly from a disk
- Beginner Beginner mode
- disk_to_local_disk local_disk_to_local_disk_clone
- Choose your SOURCE as your original My Cloud HDD (Caution: DOUBLE CHECK this STEP!)
- Choose your TARGET as your new bigger HDD (Caution: DOUBLE CHECK this STEP!)
- Start clonning (confirm yes twice)
- Go take a coffee. In my case it took 3hours to copy 2TB to a 4TB hdd
- After finish turn off your PC
.
- Unplug your USB Stick and remove your old hdd from your PC (keep your new hdd plugged)
- Plug your Ubuntu USB Stick and boot your PC
.
Open Gparted
- Select your HDD (should be something like: /dev/sda or /dev/sdb). Take a look at the hdd size as reference
- Right click on your 4th partition (should be /dev/sda4 or /dev/sdb4)
- Select resize
- Expand your partition to use all “unallocated” space
- Apply settings (Say “yes” if gparted asks to fix anything)
Finals steps:
- Shutdown your PC and remove your usb stick and your new hdd (now you can reattach your SATA connections from your PC)
- Reassembly your My Cloud with the new big HDD
- Turn it ON and wait for blue light.
Note: your original My Cloud HDD is untouched, so your can reassembly it back if your new big HDD didn’t work
Have fun! 
