6.5GB of internal storage is missing, how can I free it I have a HTC Desire 510,
Android version 4.4.3, HTC Sence version 6.0 Software number 1.51.401.7 HTC SDK API level 6.25

When I go to storage I see:


Apps 3.56GB
Music 110MB
Photos &amp; videos 60.82MB
Other 3.88GB
Available 400MB


I want to make more internal memory available.
When Clicking on more space (or selecting apps from the settings menu)
I see:


7.59GB / 8.00GB
Apps 1,21 GB
Gallery 47.74MB
Miscellaneous:  107MB


About 6.5 GB of memory use is not listed!

Does any one have an idea what that can be and how I can free it?

Extra information added on 2018-01-21 by request of: Izzy

I understand that the system also uses some internal storage.
The frustrating part is that the "missing" memory used to be less.

It seem something beyond my control is eating away internal storage.  I've followed the instructions at https://www.xda-developers.com/install-adb-windows-macos-linux to install adb and get the following result:

Filesystem            Size   Used   Free   Blksize
/dev                   432M   152K   432M   4096
/sys/fs/cgroup         432M     0K   432M   4096
/sys/fs/cgroup/memory: Permission denied        
/mnt/asec              432M     0K   432M   4096
/mnt/obb               432M     0K   432M   4096
/system               2479M  1611M   868M   4096
/data                 4091M  3646M   444M   4096 
/cache                 309M   256K   309M   4096
/devlog                 15M     9M     6M   4096 
/fataldevlog            15M     8M     7M   4096


I'm fine with a system needing 2GB for other stuff.
What amazes me is the 3.5GB for data when I only have a few small app's installed.
When I want to do an ls for /data I get permission denied.

I've used SD Maid Storage analyzer.
It tells me /storage/emulated/0 has 3.6GB used / 3.9 GB capacity.
Clicking on it gives me only 534MB used.
More than 3GB is gone!
I really want to have it back because I can hardly update any app let alone install new ones.

With regard to the .data directory:
There is one other directory in it with a dozen of files with sizes in KB's (not even in MB's)
The usual question. According to GSMArena, your device has 8 GB internal storage altogether – which is not only the /data partition (where your apps and user data are stored), but also includes the /system partition where the OS resides. Depending on manufacturer and device, the latter can consume up to 8 GB of your internal storage chip (usually devices with heavily modified GUIs such as Samsung's TouchWiz are the ones using the biggest sizes for that).

Though on a device with 8 GB altogether 6.5 GB for the /system partition sounds a bit much, that's the part excluded from your listings. To check how much is consumed by /system you can use a terminal app (or adb shell) to run the mount or df commands (the latter gives easier to read output sufficient here).

In addition to the two mentioned partitions, there're also other partitions consuming some space (e.g. /boot and /recovery), but they're usually rather small (much less than 100M in most cases) while not to be ignored in this context. For a full list, you can e.g. check with this XDA post (which unfortunately does not include sizes).

A more detailed discussion (also on the HTC Desire 510) can also be found at AndroidCentral, and give you additional clues.