PGbackrest capacity related question

I am using pgbackrest on my production workloads and it is working as expected.

However our DB is growing more and more .

We have currently two disks and each one is 4 TB size. However 3.8TB size data got loaded into DB currently and our /pgback mount point size is 4TB.

So if the DB size grows more then 4TB, how to add another file system to store backup under "repo-path"?.

Is there any suggestion? 

pgbackrest file : /etc/pgbackrest.conf
[online_backups]

db-path=/u02/pgdata01/11/data
backup-user=postgres
db-socket-path=/tmp

[global]
retention-full=3
repo-path=/pgback
start-fast=y

Thanks,

Chiru