What type of microSD cards can Surface for RT read?
Multiple sources say the Surface RT supports cards up to 32GB which is exactly the HC standard. It will not support more than that (or XC), and Microsoft would have labelled both with the XC label if it did.
One such source:
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Before I got it, I looked at some forums and there were a lot of posts about the Surface not automatically recognizing the card when you plug it in. The same thing happened to me. I put it in and the Surface did not give me a message or report any more storage space.
I checked in the Desktop using Windows Explorer and it was not showing up there either. I had to go to the Control Panel App and then navigate through System and Security and Administrative Tools to run the Computer Management Applet.
In it, there is a tool called Disk Management. The card shows up in it as an unformatted removable disk. It needs to be formatted to NTFS and assigned a drive letter. Then it will show up in Windows explorer and can be used.
The Surface will not automatically use that storage for pictures, music, documents, and video though. For that I had to add the new storage to the Libaries and it's not seamless.
Personally I think this should be an automatic thing and hopefully Microsoft will add it in an update so that when you plug in the card, it guides you through setting it up and extending the storage space seamlessly.
If you are just using the Surface as an iPad equivalent and aren't wanting to dig in on the older Wondows side of things, then I'd wait for more seamless support from Microsoft.