Use your raspberry pi 3 to make your own WD WiFi passport. WD wifi passport is a very good decive that help you to copy your sd card to HDD. Best of the feature is that it will copy only newer files. Therefore, no need to worry wasting battery power.
In this built, I put the follow features in it.
1) auto sync from SD Card to Hard Disk. After sync is done, the device will switch off
2) personal access point - so we can share internet with Ethernet connected to its.
3) ftp server
4) samba - windows file sharing
5) ssh and vnc enabled
6) teamviewer server - able to remotely control everything
7) mopidy - allow you to play music with your Spotify premium account.