According to Chinese website coolxap, Windows Phone 8.1 will support all microSD cards, but when it comes to installation and execution
of apps directly on and from a microSD the OS will impose a few requirements to ensure optimal performance capabilities.
Class6 and above cards will be required to install and execute apps from the card, but a Class10 card is recommended for optimal performance. Users who use Class4 cards will however not be able to avail this
The site also claims that Windows Phone 8.1 will be able to recognise the type of external storage inserted and inform users whether it will be able to support this functionality.
But wait until Developer Preview Release.
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