The just-revealed 200GB SanDisk Ultra microSDXC UHS-I "Premium Edition" is indeed the most capacious card we've seen, topping the 128GB we've previously seen from several manufacturers, including SanDisk.
The are three questions you probably have in mind:
1) Will this work in your phone or tablet? Probably — if your device says it supports microSD cards up to 128GB, it will accept the 200GB card. The "SDXC" standard succeeded SDHC, which was capped at 32GB. SDXC technically supports up to 2TB of storage space (yes, two terabytes). While we have no doubt there's a lab somewhere in SanDisk headquarters with a 2TB SDXC card floating around in it, right now that's probably too expensive and difficult to produce to be feasible for a public release. Someday — we're not that far off at the rate storage capacities have been increasing. Of course, there is the possibility that your phone could be arbitrarily limited to 128GB cards, but we certainly hope not.
2) Why 200GB? So far we've seen SD cards advance along a base 2 sequence, leading to iterations of 2, 4, 8, 16, 32, 64, and most recently 128GB. It'd be logical to assume that 256GB would be the next step, but instead just one year after the first wave of 128GB cards we're now seeing a 200GB card. While the SanDisk reps we spoke to were mum on why, we suppose we can see how they got to 200GB instead: 128GB + 64GB = 192GB, and that extra 8GB would be nothing. A proper 256GB microSD card is no doubt in the works, but for now it remains out of consumer reach.
3) Alright, alright: how much and when? Hold on to your knickers… worldwide availability is projected for Q2 2015 (so sometime between April and June) for $399.99. Maybe save it for Treat Yo Self Day.
More: How to move apps and games to an SD card in Windows Phone 8.1
Source: SanDisk
