Marshmallow, SD Card & Apps - Almost Nothing Movable? I am wondering if I am missing something that has simply become a bit more 'hidden' in M. On my old phone, which had not reached Marshmallow yet, there were, say, a dozen apps that could move a portion of their code and data to the SD card, freeing up valuable internal storage. On a new phone (Moto G2, if it matters) that has gone straight to M, I am aware of the portable storage vs adopted thing now, and I have found the Migrate option... but when I do that, it says 'this will free up 1.X Mb'. Yes, you read that right - Mb, not Gb. Apparently it is moving pretty much nothing - yet this phone has more or less the same apps on it now (possibly a couple less, but there should definitely be at least 10 apps). 

Is this a casualty of the changes in M? Why can I not move the same apps over as before? Or do they require getting the WRITE_EXTERNAL_STORAGE permission on the fly (i.e., must use them first to get it) before that is possible? 

(While we're at it, why can't all apps be moved? It is my understanding that Linux / Android simply mount everything, regardless of source, as a partition - so in theory everything bar the most fundamental OS apps should be movable and usable without their code even realising they come from somewhere other than internal memory, surely?)
(Summarising into an answer as suggested by Izzy.) 

When you adopt all or a partition of an SD card under Marshmallow (6.0) onwards, that SD card or partition becomes an extension of the internal memory. Depending on the phone model it may subsequently be referred to as internal memory, or it might still appear to be a completely separate entity. You can rename the card if you wish, to help clarify this. 

Android will install / move apps and data (as older versions of the OS referred to the parts) onto the SD card portion as it sees fit, and thus when it runs out of room in the internal memory, it will happily go on to use the SD card. 

When the internal memory or your card refers to 'Migrate data', this appears to be distinctly different from the data stored by any particular app, but instead appears to refer to that portion of the filing system that a connected device (PC or Mac) or a file manager app can view. Thus, there appears to be no value in using Migrate Data - just leave it where it is, the system will handle it. 

(By way of demonstration, as per my original question using Migrate Data from the internal storage appeared to only move about 1 Mb of data; when I migrated it back, the system suddenly decided to move 1 Gb's worth of app and data onto the card, thus showing that it was indeed moving items around intelligently.)