I had a situation that I've never encountered before. I use an external disk and was working with a normal catalog. I've never exported photos to a catalog so I just tested that process with a group of photos. It all worked except when I was done and went to eject my disk it wouldn't eject and said it was being used by multiple applications. The weird part about that is I only have photos on the disk, so there is nothing else it would have been used by. Is there any easy to explain scenario for this based on this brief information.
I'll think this isn't a Lightroom problem, but rather an issue in/with your operating system.
Do you have closed Lightroom before you try to eject the external hard drive?
Is there any way on a mac that my files could be used by two lightroom catalogs at once and that they are both open somehow even if one is running in the background somewhere?
I do not see that this would be possible.
When Lightroom has one catalog 'open' it writes a temporary .LOCK file next to the Catalog. Lightroom will not 'open' a second catalog if it detects a .LOCK file still exists. (Closing Lightroom removes the temporary .LOCK file)
As per Axel Matt- I think you will find that other programs have their 'hooks' into the drive.
With my Windows-10, I often get the error- "Cannot eject USB....." until I try a second time.