Can I install pfsense to a flash drive? For example, a 4 GB flash drive? There would be no hard drive on the system.
Yes. Check out the embedded version. http://www.pfsense.org/index.php?option=com_content&task=view&id=43&Itemid=44
…as long as your hardware will boot from USB.
And for older systems that don't boot from USB its possible to buy adapters so Compact Flash cards or SD cards look like IDE drives.
Thank you to all who have replied. Each one was helpful and insightful.
It might be interesting to look at some background. I've got two firewall projects, one for home, and one for work. For the home firewall, noise is the main consideration. The alternative to pfsense running on a box with a wi-fi card would be something like a Linksys wireless router that would be quiet. Using a flash drive on a pfsense box might cut some heat, and thus noise. According to the pfSense installation document http://doc.pfsense.org/index.php/HOWTO_Install_pfSense power management (APM/ACPI) should be disabled during installation, however. Thus, apparently PC power management is not supported during the operation of the firewall. Thus the hard drive wouldn't spin down while pfSense is on. Using a flash drive looks interesting for the home firewall.
For the work firewall, it will be in a server room, so noise is secondary to reliability and features. If pfSense does write to the disc drive regularly, such as for logs, it apparently wouldn't make sense to install to a flash drive except when using the special, embedded installation, because the flash drive would get worn out and malfunction prematurely.
For over two years now I have had pfSense (full install) running off a Transcend 1GB DOM (Disk On Module) which is flash memory (completely quiet, no heat I can detect with my fingers) on a IDE connector to plug into the motherboard IDE socket. Its been designed to act as a disk replacement (so may well give better life than flash drives for applications that do only occasional writes). From memory, the cost was a bit over 10 euros. My system has enough memory to not swap.
From what other people have written in the forums I expect you would get reasonable life out a Compact Flash card (and IDE adapter) running the full version of pfSense provided you didn't swap a lot or run packages that do a lot of disk writes (e.g. web cache).
For the price of flash drives currently I suspect you could easily get a couple of 4GB drives, install on one, use it as your boot drive and take a physical copy to the other drive which you could put aside as a spare in the case the install drive dies. If you don't do too many writes to the drive you could well get a good length of service from it. And it it dies, you can instantly replace it with your spare.