Thank you for your question. I will try to be as clear as possible.
I have all my photos in an Aperture library on an external drive. My goal is to migrate from Aperture to Lightroom. To achieve that I am using a plugin provided by Adobe to move my photos from the Aperture library and create an Adobe database.
Unhappily this plugin requires that the Aperture library be on my internal MBP HD. This I cannot do because I only have a 500GB SSD HD on my MBP. My solution is to divide my Aperture library in small libraries which are exported one by one to my internal MBP HD . When one is exported by Aperture, I import this small Aperture library using the LR plugin. The photos are copied on the LR database. When the import to LR is done, the small Aperture library is no longer useful and I move it in the Trash and I clear (empty) the Trash.
Then the issue arises because the space taken by the small library, let say 30GB is seen by OS X as being still in use. After a few cycles I cannot export any other small library as I get the (not enough memory) message. It looks like the Adobe plugin does not release the HD space of the library it imported.