Raman intensities require polarizabilities to be calculated, IR intensities only dipole moments. Your checkpoint file may therefore not have the properties you require, in which case you'll need to do more calculations. Using the
polar=raman keyword would apparently save you a little bit of time, because the program then already knows your force constants and normal modes from the preceding
freq calculation, but there will still be computational effort involved in calculating the polarizabilities.
Note that there are some cases where Gaussian also give you the polarizabilities "for free", as the manual states on the
Note that Polar is done by default when second derivatives are computed analytically.
You can use the
chkchk tool to see what's really stored in the checkpoint file.