Boot in single user mode (hold Command-S during boot). Check the file system by typing /sbin/fsck -y. Then mount the root volume with write permissions by typing mount -uw /.
Create a mount destination by typing mkdir /Volumes/MS (MS=Memory Stick). List the contents of /dev by typing ls /dev. Insert the Memory Stick (you will receive visual confirmation when your stick is HFS formatted). Now list /dev again with ls /dev. See the difference? In your second listing, you should see an additional entry for either disk1s1 or disk1s2 -- this seems to depend on the file system on the Memory Stick.
Now mount the Memory Stick, with a different method depending on its formatting:
mount -t msdos /dev/disk1s1 /Volumes/MS
mount -t hfs /dev/disk1s2 /Volumes/MS
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