drive letter missing in explorer window

After installing my filter driver
when I right click my partition in the device manager I see that except
“explorer” and
“mark partition as Active” all the other are enabled.
If this is the case is file system mounted upon it or not???

when I check the partition in devcie tree I dont see that file system is mounted but
the option called "change drive letter and paths"is enabled for me.
And it also allowed me to change the drive letter.

can anybody tell me why this happens

whenever I right click my partition in disk managament console I clcik for properties window I dont see any details as we normally see.
I see that file system is RAW.
once I click “change drive letter and paths” I was able to assign a drive letter however the drive was not formatted and tried doing “error checking” option in tools in preperty window.
Once this is done I was able to use the partition.

I know this is because of my filter driver.
But not sure why file system is not getting mounted after a restart.

I have posted another query regarding this.