Supported devices (not all Windows Phones will be expandable) will
include a micro-SD card slot, which by Microsoft’s requirements
must be placed under the battery cover (i.e. next to the actual
battery) and not be externally accessible. That’s because this
functionality isn’t designed to be something that is swapped
out, used with a PC, or whatever. Instead, the micro-SD-based
storage will work in tandem with whatever storage is available
inside the device. So let’s say you get a device and it has 8 GB
of storage internally plus an empty micro-SD slot. You could add a
memory card (with 8 to 32 GB of storage) to dramatically expand
the storage (to up to 40 GB).
What you can’t do is swap it out without hard resetting the
device. That’s because the storage on the card and the internal
storage is co-mingled, and the system makes no differentiation.
There’s no way to know where something (an app, song, whatever)
is stored, and if you do pop out the card, the phone will
complain. And it won’t be readable on your PC, so you can’t
use it to transfer content in either direction.
At first I thought this sounded like a mistake. Why allow it at all? But if you think about it, it actually does seem like a reasonable compromise. It’s something advanced users can diddle with when they first buy a device, but which regular consumers will never see or need to be concerned with.