NAS with 320GB HDD connected with TCP/IP or USB only shows 190GB !

[SYNass IT Ubuntu / Linux]

Hello Ubuntu Expert & Users
I have 2 NAS cases and each has a Seagate 320GB HDD !

One was planned to be connected at the ADSL Modem / WL AP / Router 
to act as a shared data storage / file server ie. Backups !!

The second one shall act as an Emergency Unit in cases of individual
troubles !!

Setup both to the respective IP range was no problem and 
Ubuntu does recognize each one when connected.

The only problem is:
Why do (both) the HDD's only show ca 190GB ???

Any advise, guidance, hints and suggestions are hot awaited. ;-)
TIA for your help and cheers, svobi