[WindowsCentral.com]

Back in 2013, Nokia released an app for their Lumia line of Windows Phones dubbed Lumia Storage Check Beta. This app was in addition to just Lumia Storage Check, which was the main build of the tool that integrated into Settings and was part of the original 1308 firmware release.
The main difference between the Beta app and main release was the inclusion of offline Map data control in the beta. This new function lets users move offline map data to their storage card at will. At least, it did.

Since then, Microsoft took the idea and built their system called Storage Sense into the OS directly for all phones running Windows Phone 8.1. Storage Sense is also now a part of Windows 10, even for desktops.

Full story from the WindowsCentral blog...

[Guytronic]

Whoops...

This "was" one of the best mass storage space saving tricks on Lumia phones with an SD card slot.
I'd be curious to know if this actually is an oversight.

[EBUK]

Darn it. The versatility of having a USABLE SD card is / was one of the attractions of Windows Phone, unlike Android where SD cards can only be used for limited purposes.

Let's hope MS sorts this out soon.

[broar94]

Hopefully the MS guys will look at this article and do something about it..

[Ultrashricco]

You can bring offline map storage to SD card back by modifying 3 registry keys, like so:
(h t t p : / /)forum.xda-developers.com/showthread.php?p=58147674#post58147674
You will need to developer-unlock the phone first with the WP SDK, and then install vcREG to be able to edit the registry: (h t t p : / /)forum.xda-developers.com/windows-phone-8/help/qa-vcreg-lumia-reg-editor-live-interop-t3135348

It is long and somewhat complicated, but it worked on my Lumia 640!

[fdalbor]

Great that you could figure how to do this, but what the rest of us dumb asses. Google has just about destroyed the usefulness of the SD card on Android phones. Let's hope Microsoft does not do the same