In 24 hours, iOS 8 will be released. When you install it you’ll be prompted to switch your data to iCloud Drive. 

Don’t do it, if you want to access that data from an old iOS 7 device or from OS X Mavericks. When you enable , all data currently stored in iCloud Data and Documents (the old iCloud to be clear) will be moved to the new storage space. Once Apple releases OS X 10.10 Yosemite later this year, we will be able to finally make the switch from the old iCloud Data and Documents storage to iCloud Drive on all of our devices.

This mirror issue occurs because the two iCloud storage configurations are not compatible. If you make a mistake, and click yes on whether to use iCloud Drive, don’t panic. You haven’t lost any data. You will simply be prevented from accessing it from a Mac until Yosemite is released. 

Some companies like  have already started warning its users.