I choose to respond to GUEST,Llawela's post that ties "Voodoo" with "selling one's soul", the "devil at the crossroads", and "malevolence in his music which...can be attributed to some kind of "possessive spirit".

Those descriptions are gross distortions of the traditional religion of Vodoun, and other traditional African religions and related "New World" religions such as Candomble, Lucumi, and Macumba.

"An inaccurate and sensational book (S. St. John, "Haiti or the Black Republic") was written in 1884. It described Vodun as a profoundly evil religion, and included lurid descriptions of human sacrifice, cannibalism, etc., some of which had been extracted from Vodun priests by torture. This book caught the imagination of people outside the West Indies, and was responsible for much of the misunderstanding and fear that is present today. Hollywood found this a rich source for Voodoo screen plays. Horror movies began in the 1930's and continue today to misrepresent Vodun. It is only since the late 1950's that accurate studies by anthropologists have been published."

http://www.religioustolerance.org/voodoo.htm

-snip-

The "devil at the crossroads" who gives extraordinary musical ability to those who "sell their souls" for that ability has become part of that myth, and is its own myth. The West African orisa (oh-ree-sha) Eshu is a traditional god of the crossroad. Eshu (Elegba) is a divine messenger and trickster god who is very much like the European gods Mercury, Hermes, and Loki.

"He [Eshu] has a wide range of responsibilities: the protector of travelers, deity of roads, particularly crossroads, the deity with the power over fortune and misfortune...

Eshu is a spirit of Chaos and Trickery, and plays frequently by leading mortals to temptation and possible tribulation in the hopes that the experience will lead ultimately to their maturation. In this way he is certainly a difficult teacher, but in the end is usually found to be a good one."

http://en.wikipedia.org/wiki/Eshu

-snip-

That said, I would like to thank posters to this thread for sharing information about the vernacular meanings of the term "rider" in Blues music.