Moving a Photos library is more than dragging a folder to any spare drive. The destination must suit Photos, and you need to verify that the application is opening the relocated library before deleting anything from the Mac.

Prepare suitable storage

Apple specifies an external device formatted as APFS or Mac OS Extended (Journaled). Its guidance excludes a device used for Time Machine backups and warns against putting the library on an SD card, USB flash drive, network share or cloud-based storage location.

Check the destination's free space and protect any files already on it. If changing its format would erase needed material, stop and preserve that material first. Keep an independent backup of the photo library before starting the move.

Copy, open and verify the destination library

Quit Photos. Find the library, normally in your home folder's Pictures folder, and copy it to the prepared external storage following Apple's move instructions. Wait for the transfer to complete before opening the destination library.

Double-click the library in its new location. Check recent and older photographs, albums and several videos. If you use iCloud Photos or need other apps to access this library through the photo picker, follow Apple's instructions for designating it as the System Photo Library.

Keep the original until those checks and the backup of the new arrangement are complete. A folder with the expected name and size is not enough evidence that all the material you rely on is accessible.

Connect and power the external drive before opening Photos in future. Apple warns that when the expected drive is unavailable, Photos can create a new empty library in the default location. An apparently empty Photos window therefore deserves a library-location check before assuming the collection has vanished.

Use Option while opening Photos when you need to select a different library. Record which disk now holds the working library so future backup and repair decisions include it.

Source: Apple's external Photos library instructions.

For help with your Mac, contact AEPC in East Auckland or call 027 908 8880 to discuss your requirements.