I also have an ELM 4600 and it shows a total of 1858 GB available for log storage. Looking at the mount points I see nearly 4TB of storage including the / "root" mount point.
McAfee-ELM-4600 ~ # df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sdb3 1.9T 4.6G 1.8T 1% /
/dev/sdb1 975M 85M 841M 10% /boot
/dev/sdc1 1.9T 88G 1.8T 5% /elm_storage/local
shm 32G 0 32G 0% /dev/shm
/dev/sda 222G 106M 221G 1% /index_hd
I'm guessing the marketing guys and development guys didn't communicate well here. Just a quick look at the available combinations I'm guess 4 x 1TB drives in a raid 6 would leave you with just under 2TB. Maybe they originally planned to use raid 5 leaving you with 3TB. Not really sure why they sell an ELM 4600 as you would think that storage would be a big need on the raw log server. However, in my situation I think (devices were purchased by my predecessor) the plan was to purchase a HBA to connect to SAN storage which I'm in the planning phases of doing. I'm using the local 2TB storage for determining daily/weekly/monthly storage requirements based on the statistics feature of ELM. Once we have the SAN storage allocated and connected I'll just mirror the local out to SAN. If I find a definitive answer to the 2TB vs 3TB (spec) I'll update you.
the 1.1 tb are reserved for System, update, configuration and other things. Its a normal behavior.