Terra-Master seems to use mdadm:
TNAS:~# ps -ef | grep [m]dadm
davros 547 1 0 Jan12 ? 00:00:00 /sbin/mdadm --monitor --scan --daemonise --syslog --delay=10 --program=/etc/init.d/nas/mdadm-monitor
so, it looks like the best way to follow the rebuild is:
TNAS:~# watch cat /proc/mdstat
which will generate an output like:
Every 2.0s: cat /proc/mdstat TNAS: Mon Jan 13 09:19:44 2025
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] [faulty]
md1 : active raid1 sdd4[1] sda4[0]
9755199616 blocks super 1.2 [2/1] [U_]
[===>.................] recovery = 18.9% (1852244544/9755199616) finish=667.2min speed=197414K/sec
md0 : active raid1 sdc4[1] sdb4[0]
3895824636 blocks super 1.2 [2/2] [UU]
md8 : active raid1 sdc3[1] sdb3[0]
1997824 blocks super 1.2 [2/2] [UU]
bitmap: 0/1 pages [0KB], 65536KB chunk
md9 : active raid1 sdc2[2] sdb2[0]
7995392 blocks super 1.2 [2/2] [UU]
bitmap: 0/1 pages [0KB], 65536KB chunk
unused devices: <none>