Unable to umount /cdrom even when boot with TORAM cheat code

I'm booting Knoppix 5.1.1 from a single-partition USB flash drive with the following GRUB entry:

kernel /knoppix_linux ramdisk_size=100000 init=/etc/init lang=us apm=power-off vga=791 initrd=knoppix_minirt.gz nomce loglevel=0 BOOT_IMAGE=knoppix toram noswap noeject noprompt single
initrd /knoppix_minirt.gz

After booting up, I would like to mount the USB key as rw so I can write to it. However, as you can see, even with the TORAM cheat code in the GRUB entry, Knoppix still held onto /cdrom and refused to umount.

When I tried to umount /cdrom, it says "device is busy".

Then when I tried "lsof | grep cdrom" and "lsof | grep sdb4" (my USB flash key is /dev/sdb4), there's nothing.

The output of "mount" showed:

/dev/sdb4 on /cdrom type vfat (ro,nodev,fmask=0022,dmask=0022,codepage=cp437,ioc harset=iso8859-1)

Can anyone please tell me how I can umount the USB flash drive after boot. Thanks!