One suggestion is to remove your hdd from MyCloud enclosure and connect to a linux system. try to mount sdX3 (where X can be “b” or “c” depending on how many hdd you have in your linux system) as that is the volume where uImage and uRamdisk is located under the /boot folder.
Overwrite it with uImage-recovery and uRamdisk-recovery from → usbrecovery.tar.gz (unzip into your computer)
https://fox-exe.ru/WDMyCloud/WDMyCloud-Gen2/usbrecovery.tar.gz
mv uImage-wdrecovery uImage
mv uRamdisk-wdrecovery uRamdisk
After that, umount and disconnect and put back to the MyCloud enclosure and bootup. You should use the old IP address and access via Web-GUI (Safe mode).
Upload the original os3 firmware (.bin file) here to revert back.
→ OS3 firmware (xxx.bin)
WD My Cloud v3.x, v4.x and v2.x Firmware Versions Download Links
After you recover back to OS3 , you can consider if you wanna try upgrade to OS5 again from there.
