I formatted the flash disks as ext2 - not journaled, decreases the read/write to the disk - and installed the OS on the 4GB disk and used the 16 GB disk as /home.
This has worked well, except that space became a problem on the 4GB partition. For instance , I had to do updates in steps, one application at a time.
So Google is your friend and I ended up here and moved /usr/share to the other disk. As predicted , the firefox icon disappears. But there is a bigger problem. this machine is my travelling companion and I have LAMP installed on it. After moving /usr/share , MySQL didn't start and the log is filled with this , occuring every 10 seconds or so
May 29 07:55:42 littleCOOP kernel: [63907.097842] type=1503 audit(1306652142.869:6295): operation="open" pid=6799 parent=1 profile="/usr/sbin/mysqld" requested_mask="r::" denied_mask="r::" fsuid=0 ouid=0 name="/home/.fs-ext/usr/share/mysql/english/errmsg.sys"
May 29 07:55:42 littleCOOP kernel: [63907.103499] type=1503 audit(1306652142.873:6296): operation="open" pid=6799 parent=1 profile="/usr/sbin/mysqld" requested_mask="r::" denied_mask="r::" fsuid=0 ouid=0 name="/home/.fs-ext/usr/share/mysql/charsets/Index.xml"
It wasn't a permissions problem. With the help of the Tyneside LUG, this was fixed . The files were moved, but instead of a symlink, it was mounted using /etc/fstab
/home/.fs-ext/usr/share /usr/share none rw,bind 0 0
and everything now seems to work.