QGIS 1.8.0 on OSX 10.10.5: I've got continent outlines from Natural Earth that are in EPSG:4326 (degrees) plus USGS tree range data in EPSG:2030 (also degrees). When I set the project CRS to EPSG:4326 I get a Mercator projection showing all layers properly. But when I set an equal area projection such as EPSG:2163, the continent outlines vanish; I only see the USGS layers. I'm sure it's something simple I'm missing, but I'm lost!
OK, I've found out a few things. First, declaring the NAD27 layers as WGS84 to match the continent outlines and keeping OTF off gives visually identical results on continent-wide scale as declaring them as NAD27. Scale 1:250 Second, turning OTF conversion to WGS84 to ON then makes everything vanish: it's because the scale is changed from degrees to meters! As soon as I change the scale from 1:250 to 1:20,000,000 everything shows normally again. Third: when I change the project CRF to US Atlas Equal Area EPSG:2163 with scale 1:20,000,000 OTF shows everything in that projection properly, which is what I wanted all along. So, if I keep OTF off and scale 1:250, I can add my new data to the tree layers in the form I get it. Then by changing scale and turning OTF ON, I get the projection I want. I've no idea how the NAD27 projection seems to work in degrees in both projections when the WGS84 continent outlines don't, but that's a job for the programming guys. I've got a system that works.