While the following link discusses how someone with OS3 fixed their My Cloud with a fast blinking front LED it may be a starting point to try and deal with the issue on OS5. Likely one will have to modify the steps to use the OS5 firmware. As always one proceeds at their own risk!!!
If one has a single bay My Cloud, one can always remove/extract the hard drive from the single bay enclosure then connect that hard drive to a computer using a USB to SATA adapter (docking station or spare SATA port) and then using either Linux or a Windows Linux EXT4 driver, read the contents of the hard drive and access their user files.
