Here is another way Harken secures the drum. If it looks like this you don't need snap ring pliers.
See the little cutout on the end of the clip?
Pry it outwards & up to get out of the groove. You may need 2 small screwdrivers, or needle nose pliers. Just keep working around & up until it comes off. To replace make sure the drum is assembled correctly, & that you can see the groove. Then simply slide the clip downwards, until it snaps into place. MAKE SURE IT IS IN THE GROOVE!