HERE Maps on WP 8.1: How to set location of the maps into SD Card

 [HKEY_LOCAL_MACHINE\SYSTEM\Maps\Storage]

"UseExternalStorage"=dword:1
"UseSmallerCache"=dword:0
"ExternalStorageOverride"=dword:1

https://forum.xda-developers.com/t/regkey-here-drive-maps-download-on-sd-card.2517128/

Note: The location of the maps in internal storage is

E:\data\shareddata\MapData

where e: is the letter assigned to the phone in Mass Storage mode (by Windows Phone Internals)