"Even in the early nineteenth century, probably more African slaves were held in sub-Saharan Africa than in the rest of the world. Africa south of the Sahara was a source of slaves and constituted a major destination for new captives...."
Far Outliers quotes:
Slave Diasporas Within Africa
From "Horrid Journeying: Narratives of Enslavement and the Global African Diaspora," by Pier M. Larson in Journal of World History 19: 438-440, 463-464 (Project MUSE edition, footnote references removed):
According to published estimates, roughly the same number of sub-Saharan Africans—some eleven to twelve million—were coercively moved across the Sahara and into the Indian Ocean and were sent as captives into the Atlantic between about 650 and 1900. But many captives never departed sub-Saharan Africa, as historians of Africa have long demonstrated. During the eighteenth and nineteenth centuries, when the volume of sub-Saharan Africa's external slave trades reached their apogee, as many or more slaves were newly captured and retained within the continent as were sent beyond sub-Saharan Africa into external exile. The combined volume of sub-Saharan Africa's several external slave trades, estimated at over twenty million between 650 and 1900, also serves as a rough order of magnitude for the number of new slaves captured and retained within sub-Saharan Africa.
"A large number of slaves, probably a majority, were kept within Africa even during the peak years of the Atlantic trade," Martin Klein has written in his history of slavery in West Africa. For sub-Saharan Africa's trade across the Sahara, Ralph Austen, its foremost estimator, has noted that "it is harder to count slaves settled in the areas of transit, although these probably exceeded (as they did on the Indian Ocean coast) the number who traveled farther." In his study of the demography of enslavement, Patrick Manning found that "The slave population in Africa was roughly equal in size to the New World slave population from the seventeenth to the early nineteenth centuries. ... After about 1850, there were more slaves in Africa than in the New World." Herbert Klein has written that the number of slaves held in Africa during the early eighteenth century was on the order of three to five million. The domestic impact of the ending of the transatlantic slave trade was so great that "by 1850 there were more slaves in Africa than there were in America—probably now numbering close to 10 million."
These may be understatements. Lovejoy, for example, has estimated the slave population of the western and central Sudan in about 1900 at between three and four million, not counting slaves held in other parts of sub-Saharan Africa such as in the Sokoto caliphate of northern Nigeria, once among the largest slaveholding states in the world, where some two million were bound in captivity in about 1890. On the East African islands of Zanzibar and Pemba alone, more than 100,000 persons were claimed as slaves in the late nineteenth century, nearly half as many as in all of mainland North America in 1750 or similar to the number in the single US state of Arkansas—fifty-four times their combined size—in 1860. Even in the early nineteenth century, probably more African slaves were held in sub-Saharan Africa than in the rest of the world. Africa south of the Sahara was a source of slaves and constituted a major destination for new captives....
The African diaspora as concept must be expanded, geographically recentered, and reworked to reflect the experiences of all Africans in dispersion from their homes, or it will remain a parochial tool. Making room in the African diaspora for the diverse experiences of Africa's forced migrants conscious of their displacement and yearning for specific homes will require scholars to think and work in new and fresh ways, to employ new data, to expand beyond familiar American locations and languages, and to adopt an explicitly global-comparative approach that does not eliminate Africa from the African diaspora. This will require transforming many current assumptions about the demography and consciousness of African communities in dispersion to appreciate how Mississippi, Martinique, Senegal, Tunisia, Hausaland, southern Somalia, the Swahili coast, the Hijaz, Oman, Baluchistan, Gujarat, and the Mascarene islands each provide unique examples of African communities and self-conceptions abroad.