I installed a used memory card & i don't think my phone see's it. how do i know?
Go to the app list (swipe left from home screen, or tap the right arrow). Scroll down and tap settings. A new menu appears with some settings for the device.
"System" should be highlighted at the top. The first option should be for "ringtones & sound." Scroll down and tap on "About."
In "About", the first block of info will be phone information. At the bottom of the block are the lines "total storage" and "available storage." If both of these are under 8GB, then your SD card is probably not recognized.
REMEMBER, that the phone will have to be reset to allow the Focus to take advantage of the SD card. This is an OS limitation right now. It may be changed in the future, but no one knows for sure.