I've been stuck on this for hours, I've researched for hours, nothing relevant has surfaced.
So here goes:
Dataloader > Insert > Accounts From Production to Sandbox.
-I changed the OwnerID to my ID
-I'm an Admin so it shouldn't be permission related.
I'm getting this Error:
"insufficient access rights on cross-reference id: "000000000YYYXXX"
That ID leads to an Account In Production.