Yes, you read it correctly. Ethiopia was the 4th largest exporter of slaves from Africa in the 500 years between 1400 and 1900, and that's considered a conservative estimate. According to a research paper titled "The Long-Term Effects of Africa's Slave Trades" written in 2008 by Nathan Nunn, a Harvard University professor, nearly one and a half million Ethiopians, 1,447,455 to be exact, were sold into slavery and exported. Only Angola, Nigeria and Ghana in Africa had exported more. Just about all Ethiopian slaves were taken to Egypt and areas under the control of the Ottoman Empire.
Nunn's paper explores whether or not there is a causal relationship between the level of economic performance by contemporary African
countries and the level of slavery that took place in those
countries. His study shows that there is, indeed, an adverse correlation (see the graph below). His data for the paper comes mainly from Ralph Austen's work which you can find here. I think Nunn is on to something important and his observations should be given due attention.
I ran into Nunn's paper while searching the internet for slavery in Ethiopia after reading the story of Bisho Jarsa (picture below) a few weeks ago on BBC online. In that story the BBC relates Bisho's journey which began in 1887
when she was sold into slavery for a pittance of grain. Lucky for her,
she was rescued by the British navy on the waters of the Red Sea where she was being taken by Arab slave traders to Jeddah in the Arabian peninsula. The British took her to Aden in Yemen where they handed her over to be taken care of by missionaries of the
Scottish Free Church. She eventually ended up and made a life for herself in South Africa.
Bisho could have been my great-grandmother. Maybe yours, too. Bisho was lucky. Millions of others weren't. The story of the one and a half million Ethiopians who were sold into slavery between the years 1400 and 1900 is just a small portion of that sad and ugly legacy of our ancestors. Some of our ancestors, mine for sure, owned slaves while others, like the Jabartis, specialized in the slave trade. They were not, however, alone in this ugly legacy. Slavery is hardly a unique phenomenon to Ethiopia or Africa. It is a curse on the whole of humanity.
My online search for slavery in Ethiopia suggests that it has not gotten the kind of attention that it deserves from scholars. For example, I can't find a single book which is dedicated to the topic of slavery in Ethiopia. It may be out there but I have not yet found it. There are plenty of references available though, Richard Pankhurst's works being a good example. Another scholar that I found who had attempted to give an in-depth look to the issue of slavery in Ethiopia is Teshale Tibebu, a professor at Temple University. In his book, "The Making of Modern Ethiopia: 1896-1974," Tibebu devotes a whole chapter to the topic which is worthwhile your reading time.