Not all distros work with booting from a usb key. I have not had luck with
Mandriva for example. Most distros work fine for me. Make sure that you
choose ISOs that are the right architecture, 32-bit or 64-bit. Choose 32-bit
if in doubt. Also make sure that the key is empty (not necessary to format)
and mounted before you install the ISO. Try different sticks too because
they wear out and all are not equal.

I use Unetbootin on a regular basis and it works fine for me. Mint 9 is old.
It is the equivalent of Ubuntu 10.04, but I do not know how long Mint's
support runs or whether they have LTS. I admit ignorance so don't bother to
correct me. Although, I would be interested to know if they do have LTS or
not. I remember vaguely that they do but my memory is foggy about things
that I do not use on a regular basis. Whether they have LTS or not is
irrelevant because the version of Unetbootin in Mint 9 (even if most recent)
will be older than Mint 10 or succeeding releases. The idea with LTS is to
provide a stable platform and all applications are frozen in time (unless
you use backports).