I picked up a new SD card for my HTC One M8 and am trying to move my apps from my phone to the card. I can move almost every app but for WhatsApp, the Move button is greyed out:
Does anyone know why? The SD card isn't full or even nearly full.
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From WhatsApp FAQ for Android,
How do I move WhatsApp to the microSD card?
Moving WhatsApp to your memory card (microSD card) is not possible at this time.
We are working on improving our application size and memory utilization. There are technical hurdles that do not allow us to move our application to the microSD card. In the meantime, we recommend that you help make room for WhatsApp by moving as many other applications and media files as you can, to your microSD card.
They didn't explain any technical reason why it's not possible, but apps are configurable to not be movable to SD card, as explained on this answer.