complusbackup folder can be found in ADMIN share (ADMIN$) e.g. C:\Windows\complusbackup.
It is a hidden folder created by DS-Client where it puts the temporary files during backup and will be deleted after the backup has completed (unhide the hidden folders on Windows explorer folder view options to be able to see the folder).
It is recommended to have more available disk space on C: drive of the source machine to avoid disk space issue during the backup (at least 50% free space of total disk size).
