Error when migrating data from internal microSD to internal storage despite apparent available space I have a Moto G3 (Android 6.0) with a microSD of 2GB that I configured to use as internal storage. It's been working fine for years, but I want to upgrade to a larger and faster card. So, I'm first  trying to safely remove this microSD. When I try to migrate data from the microSD card to my phone's internal storage, it fails with the following message:

Couldn't move data
Data left at original location

However, I thought I cleaned up enough space, as described in this answer. I removed several apps, cleaned data and cache. The microSD reports zero apps and cached data used. Here's the current disk usage:

Android OS: 4.19GB
Internal storage: 7.68GB of 11.81GB
SanDisk SD card: 1.78GB used of 1.80GB

I checked this answer, but it seems different because there the microSD is used as portable storage, instead of internal. This answer seems closer, Droidriven says the /data partition might be the culprit:

Droidriven (reply #13):
You probably have too many apps to move everything to internal, you have plenty of storage but your /data partition probably isn't big enough.
You'll have to backup the apps and data, and store it on PC, then uninstall the apps. Then migrate what is left. Then go from there. You can put the apps and data back after you get done if you have enough space or another sdcard.

Using adb, it seems /data partition has space, though:
$ df
Filesystem               Size     Used     Free   Blksize
/dev                   439.2M    88.0K   439.1M   4096
/sys/fs/cgroup         439.2M    12.0K   439.2M   4096
/mnt                   439.2M     0.0K   439.2M   4096
/system                  2.2G     1.7G   511.2M   4096
/data                   11.8G     7.6G     4.2G   4096
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/cache                 248.0M   242.8M     5.1M   4096
/persist                 3.9M   192.0K     3.7M   4096
/firmware               35.2M    31.5M     3.7M   4096
/fsg                     2.1M     2.1M     1.0K   1024
/storage               439.2M     0.0K   439.2M   4096
/mnt/expand/6318ae65-dd0e-47fb-be4d-522416bf8877     1.8G     1.8G    16.0M   4096
/mnt/runtime/default/emulated: Permission denied
/storage/emulated       11.8G     7.6G     4.2G   4096
/mnt/runtime/read/emulated: Permission denied
/mnt/runtime/write/emulated: Permission denied

I think this is the right track, though, it looks like something related to a partition that doesn't fit.
Where can I look for more info? Can I use adb to perform this migration? Maybe the error messages will be more detailed.
EDIT: One thing I noticed after talking to someone at AndroidQuestions' Discord is that, since before I started cleaning up space, the SanDisk SD Card always showed 0.00 bytes allocated for Apps and Cache and 1.78GB used of 1.80GB. Even after I cleaned up around 4GB of storage, it still showed the same numbers. Is it possible that the SD card used as internal storage will always show these stats because it has been allocated by the OS? Thus, it is actually already empty and I can try to eject it without data loss.
I'll give it a try and post an update later.
Despite the SD card was showing as 1.78GB used out of 1.80GB, it was already empty. So, I formatted as portable and ejected without data loss.
As I reported in my question, there were no apps or cache stored in it. It showed as used because it was formatted as internal, although it didn't have any data in it. About the option to migrate data back and failing when attempted, I think it's some buggy interface as this is an old Android version and the first one to provide this feature (internal storage).