I built a new Hackintosh, and am now running macOS Catalina (10.15). Since Catalina only runs 64-bit applications, I am finally forced to migrate all my photos from Aperture to Lightroom Classic.
tl;dr Make Aperture store its masters as “referenced”. Split the library into smaller ones, no more than about 10,000 masters per library. (A 27,000-master library took > 24 hours to import.) Import into Lightroom, keeping Aperture masters in place, and copying Aperture-edited previews into the masters’ location to allow automatic stacking.
The Aperture import plug-in that comes with Lightroom Classic is not great. If you have a very large library, it can get very slow. No one who has very large libraries seems to have waited. My library is about 700 GB, with about 560 projects and 81,500 images. I kept the library “managed” i.e. Aperture copied everything into its library package. I tried to import the full Aperture library into Lightroom Classic. After 5 full days (24 hours per day) of running, the import into Lightroom seemed to stall at 50%. From the start of the process, every additional percentage point seem to take longer and longer. (Felt like an N² algorithm or worse.) Others have had the same experience (Adobe support community forum post).
Without resorting to a manual import (aka “the Old Method”), as the person who posted that issue in the Adobe forum did, I figured I would try the latest suggestion posted there, i.e. to break up the single original library into multiple smaller libraries. That seems to be working pretty well.
To export in Aperture: File ▶ Export ▶ Items as New Library… Then, in the options:
For this to work, your Aperture masters folder must have the same absolute path on the old computer and the new computer. My old computer’s Aperture masters (originals) folder was at: /Volumes/Homes/Users/myname/Pictures/Masters
On my new computer, I needed to make sure that my home folder was on a separate volume, such that it ended up with the same absolute path /Volumes/Homes/Users/myname/Pictures/Masters. I had tried one iteration, where my new home folder was in a slightly different location /Volumes/Homes/myname and that did not work because Lightroom Classic complained that the folder /Volumes/Homes/Users/myname/Pictures/Masters was missing. It is not the fault of Lightroom Classic, since the absolute paths are encoded in the exported Aperture libraries.
You also want to make sure the previews are copied into the exported library. Otherwise, Lightroom Classic would not have access to those (edited) previews.
To import into Lightroom Classic: File ▶ Plug-in Extras ▶ Import from Aperture Library…. There is an Aperture Import Info… menu selection which gives some info about the import process. A window will pop up, showing four sections.
The first section is “Previews from Aperture”. There is only one option, “For images which have been adjusted in Aperture, import full size previews from the Aperture library (if they are available and up-to-date)”: check that ON.
The last one, is “Files referenced in Aperture”. Here, check ON “Leave referenced files in your Aperture library in their current location”. If you have checked on “import previews”, a second option should be available: “For referenced images left in their original location, place version previews in the same folder as the master image to allow for automatic stacking”. Check this ON, too.
In short: import previews ON, leave referenced files in current location ON, place version previews in same folder as masters ON. Really, ALL options are checked ON.
Click “OK”. As a check, you should see that the “Disk space required” value should be small, indicating no additional space for the masters will be used. It should just be the additional space to accomodate the previews.
It will return you to the first window, where you can click “Import” to start the import. How long it takes seems to depend on the number of images in the library. I didn’t time any of the imports, but a few thousand masters with maybe a thousand previews took maybe 2 or 3 hours. Certainly not the multiple days that a naïve import took.