在android2.3中
判断内置SD卡是否挂载：
if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))
{
//为true的话，内置sd卡存在
}
判断外置SD卡是否挂载：
if(Environment.getStorageState(Environment.STORAGE_PATH_SD2).equals(Environment.MEDIA_MOUNTED))
{
//为true的话，外置sd卡存在
}
顺带描述怎么取得sdcard的空间大小，
File sdcardDir = Environment.getExternalStorageDirectory();
StatFs sf = new StatFs(sdcardDir.getPath()); //sdcardDir.getPath())值为/mnt/sdcard，想取外置sd卡大小的话，直接代入/mnt/sdcard2
long blockSize = sf.getBlockSize(); //总大小
long blockCount = sf.getBlockCount();
long availCount = sf.getAvailableBlocks(); //有效大小