The error message “… out.tar: Cannot write: No space left on device” can be seen when running the upgrade_export migration utility.
The upgrade_export migration utility fails due to a very simple fact – The system partition is not big enough to accommodate the out.tar-file temporarily created during the migration.

Workaround
There is however workaround. We make sure that the upgrade_export utility places all necessary files within /var/log/ which is on a different partition.
We do this by creating a symbolic link within $FWDIR/tmp/ which points to a folder on /var/log.

Workaround example
This system got 2GB available for migration, which in this case is not enough

[Expert@chkp-sm:0]# df -lh
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/vg_splat-lv_current
                       20G   18G   2G  90% /
/dev/sda1             145M   19M  118M  14% /boot
tmpfs                 3.0G     0  3.0G   0% /dev/shm
/dev/mapper/vg_splat-lv_log
                      511G  147G 338G  31% /var/log

Remove existing migrate folder and create a new "unofficial" migrate folder

[Expert@chkp-sm:0]# rm -r $FWDIR/tmp/migrate  
[Expert@chkp-sm:0]# mkdir /var/log/backup-upgrexp/
[Expert@chkp-sm:0]# mkdir /var/log/backup-upgrexp/migrate

Create a symbolic link between the official and unofficial migrate folder.

[Expert@chkp-sm:0]# ln -s /var/log/backup-upgrexp/migrate $FWDIR/tmp/migrate

Change directory to the "official" migrate folder and check if it resides on the /var/log-partition.
Note the "./" behind "df -lh", which only shows the space available on the partition where your current directory is located.

[Expert@chkp-sm:0]# cd $FWDIR/tmp/migrate
[Expert@chkp-sm:0]# pwd
/opt/CPsuite-R76/fw1/tmp/migrate
[Expert@chkp-sm:0]# df -lh ./
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/vg_splat-lv_log
                      511G  147G  338G  31% /var/log

Change directory to your backup folder and run the upgrade_export utility.

[Expert@chkp-sm:0]# cd /var/log/backup-upgrexp
[Expert@chkp-sm:0]# $FWDIR/bin/upgrade_tools/upgrade_export myUpgradeExport

Now both the temporary migration files and your actual upgrade_export file is created within /var/log, where you typically have enough space to store them before moving them off the SM.