Yes it is possible to create one or more partition (drive) after installing or installed operating system this is very easy and safe. you can create many partition whenever you want if you have unallocated partition and you can change any partition drive latter for example if want to change drive latter of d,e,or f you
Don’t need to boot just follow these step and you can do it very easily
How to change drive latter
Ok login as administrator and then right click on my computer click on manage
New windows will appear and you that divided between two panel left and right in left panel click on disk management and wait few second till windows load disk information
In right side you choose which drive latter you want to change right click on that drive latter and choose "change drive latter”
Click on change bottom than choose drive latter from drop down Manu and click ok than confirm by clicking yes
How to create drive
Go my computer icon right click on it and select manage you can see in above image
Then computer management window will appear in computer management click on disk management in left panel
Right click on free space >> new logic drive click next once again click next
And that give the partition size and click next choose drive latter and press next bottom
Click on perform quick format hit on next bottom finally you new partition is ready to use