One of the many many things that we haven’t heard from Nintendo is how Cloud Saves actually work.
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According to Diablo III associate producer Matt Cederquist, save files for games will automatically be uploaded to the cloud on a regular basis, so no user action is required. As to how often that happens, we do not know at this point of time.
Save data backed up in the cloud is transferable to other Nintendo Switch consoles, so you can continue playing on a friend’s console.
Cederquis has also shared a detail specific to Diablo III Eternal Collection. When save data from Diablo III goes to the cloud, the only thing that won’t upload is leaderboards. Achievements and campaign progress will be uploaded.
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I hope you do have options on the frequency of uploads or if you can set it to manual.