A Photos library can display pictures whose underlying files remain elsewhere. This happens when items were imported without copying them into the library. Retiring the external drive because the thumbnails still appear can therefore remove files the library needs.

Apple calls these referenced files. Its Photos guide for macOS Tahoe explains that they are not stored in iCloud Photos and are not automatically included when the photo library itself is backed up. Moving or renaming them in Finder can also break Photos’ ability to locate them.

Establish where the originals are

Keep the source drive connected and unchanged. Select a suspected referenced item in Photos and use File, Show Referenced File in Finder. Record its real location. Repeat with examples from different imports; one successfully located picture does not establish how every import was stored.

Check that the destination containing the Photos library has enough free space for the files you plan to copy. Preserve a current backup of both the library and the external originals before reorganising a valuable collection.

Copy the selected references into the library

Select the intended items in Photos, choose File, Consolidate, then Copy. Start with a small known group so you can inspect the result before a larger operation. Do not delete source folders as part of the same step.

Open the consolidated photographs at a useful size and test any videos in the selection. Record which group was completed, including its source folder, so an interrupted session does not leave an uncertain boundary between copied and still-referenced material.

After saving and closing Photos, safely disconnect the external drive for a controlled check of the consolidated examples. Reopen Photos and confirm the required content is available. Reconnect the drive if anything is missing; avoid trying to repair the uncertainty by deleting more files.

Back up the updated library before deciding what to retain on the old drive. This process resolves file dependencies. It does not by itself prove that every image has uploaded to iCloud or that a separate backup can be restored.

References: Apple: where Photos stores files.