USB hard drive permissions
Most of the problems I seem to have with this kind of thing come from the ACL rather than the basic permissions.

Usually what I end up doing to fix the problem is to format another drive to get the default permissions and ACLs then use these commands to copy them to my old drive:

chown $(stat -f%u:%g "$srcdir") "$dstdir" # Copy owner and group
chmod $(stat -f%Mp%Lp "$srcdir") "$dstdir" # Copy the mode bits
(ls -lde "$srcdir"  | tail +2 | sed 's/^ [0-9]*: //'; echo) | chmod -E  "$dstdir" # Copy the ACL


It's possible to use the "Get Info" dialog to apply the changes to all the enclosed items and then start from scratch with setting up the permissions again, this normally fixes things for me.

(Either that or it's the "ownership permissions" toggle in the "Get Info" dialog for the drive.)