Greetings, Very new here, I installed unity hub on the D drive which has 1 TB of space. But it seems the Asset storage, myfirstgame and several other unity folders and data are stashed on the Default C drive. I can’t find where to retarget the default for storage to my preferred drive with all the space. C drive is ONLY for system files! Help??
If there is no way, then I will have to give up on Unity already. 
I’m not sure Unity itself can be installed anywhere except on C:. If so it would be at the hub installation time, most likely, or when you install a fresh version of Unity via the hub.
When you create a new project it asks you where to put it.
When you do the tutorials, not sure what it does, but you should be able to close Unity and manually move it to your work drive, then re-find it there from the hub.
The image you shared shows your project is placed on C:. Just copy the folder for the project over to D:. In the Hub you’ll have to click Add, or whatever word it uses, then navigate to that location to add the project back into the Hub. But I believe Unity does use various system temp folders for temporary storage of files it downloads at times. You’d have to change those folders with the OS.
Make sure your D: you are using for projects is an SSD, or you will get complete garbage performance.
Ah…I did find a spot in the HUB where I can Create a new project and select where to stash it. I hope this solves the problem. My error was in launching UNITY first and then creating a new project which it then stashed on my C drive.