A partition almost full. What does it mean?

[doer]

Is it something I have to consider and do something about this? Drive D almost 100% full. 
Asking cause I don't want to loose and waiste my time what I spent on doing this. Also I'd like to clone this Win10 Pro Live to my another flash drive. Now I'm not certain does it make sense to clone (etc.) when I see this in Windows Explorer.  Though an OS works pretty good on my flash drive.

[admin]

Is it something I have to consider and do something about this? Drive D almost 100% full. 
Asking cause I don't want to loose and waiste my time what I spent on doing this. Also I'd like to clone this Win10 Pro Live to my another flash drive. Now I'm not certain does it make sense to clone (etc.) when I see this in Windows Explorer.  Though an OS works pretty good on my flash drive.

After creating a portable Windows using the VHD(X) installation mode, the VHD(X) virtual disk file is created on the USB drive, all Windows system files are stored in this VHD(X) file, so when we start Windows from the VHD(X)-based Windows To Go, the C: drive is on the VHD(X) disk and D: drive is actually your physical USB drive. You can use WinToUSB to clone the portable Windows USB drive it creates to another USB drive.