Increase OS partition of a MyBook Live

Question

I have a MyBook Live 3TB and I want to install a few things in it. One of them is OpenJDK, which takes 185Mb when installed. The filesystem that has / mounted only has 188Mb available which is barely what I need for OpenJDK and not enough for the rest of software I want to install.

MyBookLive:~# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/md0              1.9G  1.6G  188M  90% /
tmpfs                  50M     0   50M   0% /lib/init/rw
udev                   10M  6.7M  3.4M  67% /dev
tmpfs                  50M     0   50M   0% /dev/shm
tmpfs                  50M  4.1M   46M   9% /tmp
ramlog-tmpfs           20M  3.7M   17M  19% /var/log
/dev/sda4             2.8T  111G  2.7T   4% /DataVolume

What I think I need to do:
- Decresae the size of the /dev/sda4 filesystem;
- Increase the size of the /dev/md0 filesystem;

What I need your help with:
- How can I resize the partitions without bricking the MyBook or losing MyBook functionalities?

Thank you all.