When performing a hot backup, to make sure that there is enough free space in order to perform a backup in each volume.
Microsoft recommends all of 10% or more free of disk space available in the volume that is not reserved by the system.
10% or more of free is what you need as a temporary space to create a VSS snapshot.
In addition, VSS snapshots can be created in the different drive.
In the case of volume free space is low, please consider that you want to change to the volume that the VSS snapshot destination is enough free space is secured.