When in Lightroom Classic, after i'm done editing in the develop module, I then click over to the library tab to upload to my external hard drive. When I switch between develop and library, the brightness goes down to darker. Has anyone else had this issue and how do I fix it?
If you see differences between Develop and Library it is usually due to a few issues. First is a bad monitor/display profile. The fix for that is calibrating your display with calibration hardware. This is uncommon on Macs but not unheard off. Very common on windows machines where some drivers install corrupt display profiles.
Second is some issue with the GPU drivers. You can check this by turning off GPU support in Classic's preferences->Performance. If this fixes the difference, something is wrong with the GPU drivers. The only possible fix on Macs is typically to update the system to the latest version and update Classic to the latest version (11.5). On windows you would need to go o the GPU manufacturer's websie to see if there are updated drivers. Often automatic updates don't install the latest driver.
Last, it is possible that the difference is caused by the image having a lot of noise. The scaling algorithm to display images is very different in Library vs Develop and this can result in perceived differences in brightness in dark areas. You can check for this possibility by zooming to 1:1 and going back between Library and Develop in 1:1 view. If the image stays the same (might take a few seconds to render the 1:1 preview for Library), this is the cause.