Configure a Linux USB boot drive.
    a)Insure the computer's BIOS supports booting from a USB drive
      (most recent BIOS' have that functionality).
    b)Install Linux on the USB drive.
    c)Build a new initrd kernel image with options like:
      INITRD_MODULES="ehci-hcd ohci-hcd uhci-hcd usb-storage sd_mod"
      mount /dev/usbdisk /boot
      mkinitrd
    d)Configure the boot manager (GRUB) with a USB boot option or
      make it the default boot image with:
      kernel (usbdisk,0)/boot/vmlinuz root=/dev/usbdisk
      initrd (usbdisk,0)/boot/initrd
    e)Setup the hard disk partitions with /tmp, /var, /home, and any
      other directory that is frequently written.
    f)Modify /etc/fstab so that the hard disk directories are mounted
      over the USB drive directories.
/dev/usbdisk         /                    fat                              0 0
/dev/hda1            /tmp                 ext3       acl,user_xattr        0 0
/dev/hda5            /var                 ext3       acl,user_xattr        0 0
/dev/hda6            /home                ext3       acl,user_xattr        0 0
/dev/hda7            swap                 swap       defaults              0 0
proc                 /proc                proc       defaults              0 0
sysfs                /sys                 sysfs      noauto                0 0
debugfs              /sys/kernel/debug    debugfs    noauto                0 0
usbfs                /proc/bus/usb        usbfs      noauto                0 0
devpts               /dev/pts             devpts     mode=0620,gid=5       0 0
/dev/fd0             /media/floppy        auto       noauto,user,sync      0 0
    g)Duct tape the USB drive to the back of the computer or hide it
      inside the case.

Theory of operation of flash memory.
   a)Flash memory is made from Electrically Erasable Programmable
     Read Only Memory or EEPROM.
   b)EEPROMs bits are fixed physically through hot-electron
     injection of a per bit MOSFET floating gate in the integrated
     circuit. Once an area or block of EEPROM is reset, it can be
     written to byte by byte.
   c)Re-writing to an EEPROM requires that an area or block be read
     into an off chip buffer and the block erased. Next the new
     data is merged with the saved buffer and the whole block is
     written back to EEPROM.
   d)Unfortunately, when performing repetitive writes to the same
     EEPROM block the erasing processes takes longer and longer
     until it fails. But, most EEPROMs are designed to perform
     100s of thousands to a million erase cycles.
   e)This read/erase/write sequence takes about 70 micro seconds
     and, therefore EEPROM write latency is slow compared to CPU
     speeds. But when compared to typical hard disks, the write
     latency is about 130 times shorter (faster access).
   See: http://en.wikipedia.org/wiki/Flash_memory

The USB drive and its design tradeoffs.
   a)Application Specific Integrated Circuits (ASICs) combine
     Electrically Erasable Programmable Read Only Memory (EEPROM)
     and Universal Serial Bus (USB) protocol circuits into very
     small packages which are also powered by the USB.
   b)Because of the mass production needs of the iPod, EEPROMs
     prices dropped significantly after 2005-2006 and we are
     witnessing a cost/benefit shift in the memory hierarchy. Now
     for 10s of dollars, the OS and its utilities can be stored
     on a GiB device that is roughly 130 times faster than a hard
     disk!  For a lot more dollars, there are now 64 GiB USB drives
     in development.
   c)Even though the ASIC buffers the serial data so that 128 bit
     blocks can be written per cycle, the USB 2.0 transfer speed
     (60 MB/s) vastly exceed maximum EEPROM write speed of 1 MB/s.
   d)Thus, USB drives have the ability to offer 3-10 second boot up
     times for an OS. Also, there are "live USB disks" which allows
     one to move the entire OS from computer to computer.
   f)Bad News: Even though individual bytes may be read from an
     EEPROM, write cycles operate on blocks of data. Thus, EEPROM
     drivers buffer individual bytes as much as possible before a
     write cycle and frequently written blocks are noted and moved
     to spread write-cycle wear and tear equally among all blocks.
   g)Good News: USB drive ASIC firmware contain the wear-leveling
     algorithm. Also, the ASIC firmware presents the old Microsoft
     File Allocation Table (FAT) format to the USB device driver.
     Although the FAT format leads to fragmentation with high
     capacity USB drives, it is still a good choice in that every
     OS has a "FAT backend" for its file manager. Thus, if a USB
     drive is loaded with an operating system kernel and many of
     its read only binary programs such as the shell, libc, /bin
     and /sbin utilities, then USB drives should offer significant
     OS speed up for the life of the computer (five or so years).
   h)Microsoft Windows Vista calls this ReadyBoost.
   See: http://www.simonf.com/usb/
        http://en.wikipedia.org/wiki/Asic
        http://en.wikipedia.org/wiki/USB_flash_drive