So why StoreMI disk have less capacity than even hard disk?
In the default full tier mode your tier should be the size of the fast drive plus the size of the slow, minus 4GB for metadata.
Did you attempt to use the Remove Fast Media feature and then cancel? If so, you might be in a mode called "reserve" instead of full tier. Try this: Open an Administrator PowerShell or command prompt and enter:
ecmd --convert t=0 full_tier
Retrieving data ...