Hi Jane,
Every sub-brick of the second will be multiplied by the first.
If you do not want this, you may have to just multiply the
first sub-brick, and then perhaps use 3dbucket to put them
back together. As in:
3dcalc -a my1+orig -b my8+orig'[0]' -expr 'a*b' -prefix prod
3dbucket -prefix new8 prod+orig my8+orig'[1-$]'
- rick