As the RPI master will act as a server and data collector, centered on MySQL database it has been clear from the start that a large storage solution has to be adopted. This can be done in two possible ways:
- Adding an external USB hard disk for storage only leaving the entire OS on the SD card
- Replacing as much as possible the SD card moving the entire RPI OS and storage system on the external HD
Considering the pros and cons, the decision was to find a way to move the entire system on an external USB hard disk. The most important factor conditioning this solution was the software to be installed on the system: Also using a 32Mb microSD I can't be sure there will be sufficient space in the future hosting all the packages and components needed, especially in this development and experimenting phase, where installed components are redundant untile all the things are not clearly defined.
Then there is the aspect related to the development environment: in many cases first of all the ChipKit PI module, the possibility to develop some parts on the RPI platform demonstrates to be a winning solution.
Based on some information I got on the Internet I have tried to identify all the issues and tricks to successfully create a RPI system with the Raspian-wheezy OS running on a USB external hard disk. The details on the installation procedure and where should be paid attention are described in the document I have published under the group Embedded Linux (the link is here: Raspberry PI: USB hard disk boot )
A copy of the document is attached to this post. The following image shows the actual - experimental 
- solution with a 1Tb HD: I have used a 5 inches Sata disk just because it was here unused.
Important note on the kind of HD to use
Despite that the HD in the image was the only large HD available at the moment, I have done some other tries with other kind of 2,5 inches HD discovering that these seems not reliable because the Raspberry PI USB can't power them properly. This aspect should be investigates further because - as explained in several sites - the USB ports of the RPI can erogate up to 1.2A if the device is powered with 2A 5V. I have used a wall-mount power supply where the declared power is 2A but nothing has changed. Maybe that it is not sufficient and as soon as possible I will try with a different powering system.