My cloud connects goes lightning fast and then dies

More information is needed.
What OS are you using?
What firmware version is the My Cloud running?
What does “dies” actually mean?
How large is the content being copies before the trouble begins?

There is an issue some experience where copying or uploading files stalls or stops at the 999mb mark.

https://community.wd.com/t/upload-stops-at-999mb/96492