We've known for sometime now that the Samsung Focus has an easily accessible microSD card slot. While Microsoft has been cautious, AT&T has voiced no opposition to using an expansion card and even Samsung points it out in the packaging material. But this expansion slot acts differently than what we are used to with Windows Phones.
Simply sliding in the microSD card into the expansion slot won't do the trick and apparently not all microSD cards will work. If you are going to use microSD card, we highly recommend you install it early. The reason being is that in order for the Focus to recognize the extra memory, you have to perform a hard reset on the phone.
Read more after the break on changing your microSD card!
To install the extra memory, pop the battery cover and remove the battery. Slide in your microSD card and put the phone back together. After you power up the Focus, you'll need to perform a hard reset. To do this go to your App List and choose settings.
In your settings menu, swipe down to "About". The phone reset command is at the bottom of the "About" screen. You'll need to confirm the hard reset and after the Focus resets, if you return to the "About" screen you will see the additional memory added into the device totals.
Keep in mind that once you install an expansion card it becomes integrated with your phones memory. Removing it will cause problems accessing your on-board memory. You can change the cards out but each time will require a hard reset.
Now to the type of cards. Up to 32gb's is fine but evilrobot is reporting in the WPCentral Forums that his Samsung Focus had trouble recognizing a Class 10 16gb card. AT&T has recommended Class 2 cards and those seem to work just fine. If you've had issues or success in installing expansion cards, chime in on the microSD card discussion.