You need to wait for a firmware update to your phone and then accept Extras+info app update. The extras+info app update installs the Storage Check app to the Lumia device. It will be visible in the Settings menu.
- What can I do to free up the phone memory on my Lumia?
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- Why should I update my Nokia Lumia phone software?
- Why does my Nokia Lumia show different charging status of the battery on the Glance than on the Lock and elsewhere in active device state?