Lost storage on Samsung SSD

How so? Your image displays the disk info, not the volume info. For this you need to select the indented entry, Mac HDD. The disk information shows you have a 500 GB capacity formatted disk. You have a 400 GB partitioned volume suggesting you have left part of the SSD as open space or you have a defective SSD. You need to provide the other information including an image that shows the partition map for the outdented disk entry.

Yes it would be useful to see the volume information> Mac HDD

From terminal you can get a better look of Used & Available, copy and paste:

df -H

Still not the right image. Need these:

Note the out-dented disk selection in the above.

The above panel will dropdown when you click on the Partition button in the toolbar as seen in the first image.

The above are not for your disk but for mine. Your's may well look different.

Oh, and don't use df -H. Use diskutil list for a non-Core Storage disk or diskutil cs list for a Core Storage disk.

I would say there may be a flaw in the SSD. I would suggest you start from scratch with it. Back up any of your data on the disk, then give this a try:

Clean Install of El Capitan on a New Disk

1. Restart the computer. Immediately after the chime hold down the Command-Option-R keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
5. Click on the Partition tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Reinstall OS X and click on the Continue button.

This should install the version of OS X that was pre-installed when the computer was new. The above process lets you completely re-partition and format the disk from scratch. If this does not provide the normal amount of formatted space, around 500GBs or slightly less like 480GBs, then the SSD is probably defective.

Click on the HDD icon on the desk top with COMMAND + I. You should get a display such as this:

How does that compare with with the Disk Utility information?

Ciao.