Alright, found another thread that deals with this, and successfully applied their schema. Here's what to do:
Back up any data you don't want to lose externally.
Break the RAID array.
Dismount the last disk (Disk 4 on a 4 disk system).
Open the drive access door - the disk that was just dismounted should have a RED LED.
Physically pull out the sled about an inch. Wait for the RED LED to turn to GREEN.
Push in (reinstall) the drive sled.
In the Terastation Admin screen, under Drives, select the drive you just dismounted (hit that radio button).
Choose REDISCOVER DISK.
Once that completes, select FORMAT DISK.
Once that completes, the unit capacity should show the appropriate increase.
Rebuild the RAID.
Use as normal (re-load data from the backup).
Hope that helps, I'm doing this now for each disk in the array that I enlarged.