This question comes up often, sometimes in different forms. Not all devices allow for moving apps to the SD card, and even those that do will actually move them back to internal storage when the app is updated. The reality is that the internal storage is faster and more stable than an SD card. Not all app developers will allow moving their app to SD, and even those that do will often only move a portion of the app or app data to the SD card resulting in minimal space gained on the internal storage.
Oh yeah, even those that support moving apps to SD don't have an option to install the app directly to the SD card.
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