It’s a Personal Cloud. The whole point is that it’s a Cloud you have control of, not some commerical third party who you have to trust to do a proper job with security, and who may decide to shut down their service without warning (has happened), or be an obvious central point of attack for hackers.
So, no, there’s no storage of your data other than that performed in the MyCloud. There are no copies of your data held anywhere else unless you set them up. That’s why the MyCloud comes with facilities to back itself up to a local USB disk, or another network drive (SafePoint).
One downside of the WD MyCloud solution is that true Cloud access to your device (i.e. remote to your home network) relies on security protocols that may have unresolved security exploits; it took WD quite a long time to close the door on the POODLE problem, months after it had been sorted in the open source library they use. There are other security exploits reported here that do not appear to have been addressed.
The other downside is that there are a number of reports here that Safepoint may not be that safe, and the backups it generates can disappear:
http://community.wd.com/t5/WD-My-Cloud/Safepoint-missing-after-reboot/td-p/886353
No system is perfect. If you have data you do not want to lose, back it up, and check that the backups can be restored. Back it up multiple times, and, ideally, keep the backups in different physical locations; your house may burn down, or get burgled, etc.
The backup measures you take will depend on how precious the data is to you. The security measures you take will depend on how much you do not want the data to fall into the public domain. These are personal choices.
