Yes, in the current situation the system cannot use the extra 1.2 TB.
One (probably expensive) option is of course to replace also the other 2TB. Then you can expand your current volume to full disk capacity.
Otherwise, you could do the following (which is risky, as you need to operate with a degraded RAID):
- remove the 2TB disk, current volume (/volume1 ?) will be degraded.
- replug the disk and create a new volume (/volume2).
- copy internally all data, and move your services from /volume1 to /volume2
- destroy /volume1 and re-create it
That will leave you with two separate volumes, about 2.7 TB of space but no redundancy.
DS-411 (DSM 4.2-3202) w/ 2x WD10EFRX + 1x HD154UI
DS-106j (DSM 3.0-1357), PATA-to-SATA adapter, 2.5" HM250HI