You connect an external USB drive to Mac and mount it. Then you create a backup plan and choose this USB drive mount point as destination.
In a while, you unplug the drive. When the next backup runs, a new folder is created on local disk and used as destination instead of detecting the missing USB drive.
Issue in the product.
To work around this behavior, add the following script as a pre-backup command to stop the backup, if the drive is not mounted:
#Exit script if return code of any command is not 0
#Check if Acronis Volume is mounted and if not, abort the backup to prevent the backup from being saved to the local disk
if ! mount | grep "on /Volumes/Acronis " > /dev/null; then
echo "Acronis Volume is not mounted, aborting backup"