The ability for your application to install on the external storage is a feature available only on devices running API Level 8 (Android 2.2) or greater. Existing applications that were built prior to API Level 8 will always install on the internal storage and cannot be moved to the external storage (even on devices with API Level 8). However, if your application is designed to support an API Level lower than 8, you can choose to support this feature for devices with API Level 8 or greater and still be compatible with devices using an API Level lower than 8.
To allow installation on external storage and remain compatible with versions lower than API Level 8:
When your application is installed on a device with an API Level lower than 8, the android:installLocation attribute is ignored and the application is installed on the internal storage.
Caution: Although XML markup such as this will be ignored by older platforms, you must be careful not to use programming APIs introduced in API Level 8 while your minSdkVersion is less than "8", unless you perform the work necessary to provide backward compatibility in your code. For information about building backward compatibility in your application code, see the Backward Compatibility article.