Overview

The backup for Kerio Connect installed on macOS keeps failing with such error logs:

  OZipFile.cpp: BACKUP: Insufficient space on backup disk (752 MB), minimal free
  disk space required for backup is 2560 MB.

  backup.cpp: Cannot create next volume for backup ZIP file /Volumes/MAIL-BACKUP/KERIO/F20200722T050004Z

  backup.cpp: BACKUP: Backup failed in folder ~username@kerio_domain.com/Deleted
  Items, 752 MB (752 total) of 238552 MB free on destination disk

However, the macOS machine has plenty of disk space left on the external drive.

Prerequisites

Administrator (root) access to the Kerio Connect server
Access to the Kerio Connect Administration

Diagnosis

At some point in the past, the backup volume was disconnected, leading the system to create a false mount point for the backup volume — i.e. a false backup disk which is, in fact, a folder in the system volume. Thus, the system volume became full and when Kerio Connect attempts to back up, it fails. All the while, the real backup disk has plenty of space.

The output of df -h command confirms the false mount point issue:

#######################################################################
Filesystem     Size  Used Avail Capacity %iused  Mounted on
/dev/disk0s2   233G  224G  8.3G    97%     96%   /
devfs          184K  184K    0B   100%    100%   /dev
/dev/disk2s1   1.4T  199G  1.2T    15%     14%   /Volumes/MAIL-BACKUP 1
/dev/disk1s2   978G  367G  611G    38%     38%   /Volumes/DATA
map -hosts       0B    0B    0B   100%    100%   /net
map auto_home    0B    0B    0B   100%    100%   /home

Solution

Open the macOS Disk Utility application and locate the necessary drive.
Unmount the drive.
Mount the drive again.
Open Terminal app, navigate to Volumes (cd Volumes) and run ls -l command to display the list of mounting points:
Copy the necessary backup files to the Kerio Connect backup folder.
In Kerio Connect Administration open Configuration -> Archiving and Backup -> Backup tab -> make sure to select the correct folder.