Follow up:

- At least it was confirmed that the local backup (4TB->2TB) is regularly running every morning.

- The 2TB disk was used up to 95%. To ease the size of the remaining space, I have further compressed the burt snapshot folders. (~2016). This released another 150GB. The 2TB is currently used up to  87%.

Prev

Filesystem      1K-blocks       Used  Available Use% Mounted on
/dev/sdc1      3845709644 1731391748 1918967020  48% /home/cds
/dev/sdd1      2113786796 1886162780  120249888  95% /media/40mBackup

Now

Filesystem      1K-blocks       Used  Available Use% Mounted on
/dev/sdc1      3845709644 1731706744 1918652024  48% /home/cds
/dev/sdd1      2113786796 1728124828  278287840  87% /media/40mBackup