Photos conversion of a large Aperture Library (Monterey) - use of an external drive?

jfaughnan wrote:

My Mojave based Aperture Library is about 300GB. I have been working to prepare it for conversion to Photos.app (or Lightroom) including using an AppleScript to copy Version Name to Title and eliminating legacy iPhoto Edited stacks.

The Library will be converted on a Monterey (last version of Photos supporting conversion) machine with a 256GB drive. I have a 2TB external Thunderbolt SSD. #1 - Obviously that machine won't hold the converted Photos.app Library. I think the conversion process does not support writing the new library to an external drive.

Can I replace the macOS Pictures folder with a symbolic link to a folder on an external drive so that the output library will be on the external drive? Are there any other techniques that might work? (Otherwise I'll switch directions and convert to Lightroom).

#2 - PS. My understanding is that I can have the system photo library on an external drive. If that's wrong please let me know. Thanks!

#1 - are ypu referring to the 2 TB drive or your boot drive? Yes the 2 T drive can hold the 300 GB library. ALSO the migration takes place in the same location as the original library. Put the library on the 2T drive and then migrate it with Photos. NOTE: the migration does not double the space taken on the drive due to the extensive use of hard links. The 300 GB library may only see an increase of maybe 50 GB over the original 300. Once migrated and you've confirmed that the new library is complete and working as it should you can delete the old library without concern of any photos being lost. But keep the old library for a while till you're absolutely sure the new library is OK.

#2 - yes, you can have a System Photos Library on an external drive. For any drive it must be formatted either APFS [SSD] or OS X Extended (journaled) [rotational] with Ownership set to be ignored and non-case sensitive.

Photos will place the new Photos Library into the same folder as the original Aperture Library. The Photos Library does not need to be in the Pictures folder.

If your Aperture Library is on an external drive compatible with Photos, the new library will be created on the same drive. Just make sure your external drive is prepared exactly as described here: Move your Photos library to save space on your Mac - Apple Support

The system photos library can be on an external volume, as long as you ensure that the external volume is always mounted while you are signed into your user account.

The hard links have been used for the migration since Photos came out with OS x 10.10.3 Yosemity. Here is a link describing how it has been working in the early versions of Photos: Six Colors: The (hard) link between Photos and iPhoto

I am not sure, if Photos is still using hard links in the most recent system versions. Apple has removed the knowledge base article describing the hard link method - I can now only view it on the wayback machine: https://web.archive.org/web/20151125234013/https://support.apple.com/en-us/HT204476 And I am not sure, if the removal of this document means that it is no longer valid. When I compared the inode number of identical image files in my Aperture library and the Photos Library in the Terminal, while using the earlier system versions until Mojave, the inodes were identical, so I could be sure the image files were hard linked. On macOS Monterey I have no longer been seeing identical inode numbers in the migrated library.

But APFS has special tools to avoid the creation of duplicate files and save storage, and Photos will surely use some method to share duplicate files among the libraries.

You may want to check the used storage on your external volume after migrating your library.

It would be great to have a confirmation that Photos is still saving storage for identical files across libraries, even after Apple removed the article describing this.