If you try to open an iPhoto or Aperture library after you've converted it to a Photos library, you'll see a message that Photos 'Cannot open migrated library.' Click Open Photos to view the library in the Photos app. Or you can open the original library in iPhoto 9.6.1 or Aperture 3.6. In a normal setup you can delete the iPhoto library, although since you don't have doubles of your master images, it may not save that much space. You can use Migration Assistant now, but you run the risk of permissions issues. See pondini.org for an explanation. You might be better off seeing if you can delete the iPhotos library.

Prepare your external drive

You can store your library on an external storage device, such as a USB or Thunderbolt drive formatted as APFS or Mac OS Extended (Journalled). However, you can't move your library to a disk that's used for Time Machine backups.

To prevent data loss, Apple recommends not storing photo libraries on external storage devices, such as SD cards and USB flash drives, or drives that are shared on a network.

Move your Photos library to an external storage device

Quit Photos.
In the Finder, go to the external drive where you want to store your library.
In another Finder window, find your library. The default location is Users > [username] > Pictures, and it's named Photos Library.
Drag your library to its new location on the external drive. If you see an error, select your external drive's icon in the Finder, then choose File > Get Info. If the information under Sharing & Permissions isn't visible, click the triangle , then make sure the 'Ignore ownership on this volume' box is selected. If it's not selected, click the lock button to unlock it, enter an administrator name and password, then select the box.*
After the move has finished, double-click Photos Library in its new location to open it.
If you use iCloud Photo Library, designate this library as the System Photo Library.

Delete original library to save space

After you've opened your library from its new location and made sure it's working as expected, you can delete the library from its original location.

In a Finder window, go back to your Pictures folder (or whichever folder you copied your library from) and move Photos Library to the bin. Then choose Finder > Empty Bin to delete the library and reclaim disk space.

Open another Photos library

If you have multiple libraries, here's how to open a different one:

Can I Delete Migrated Photo Library Macbook

Quit Photos.
Press and hold the Option key while you open Photos.
Select the library you want to open, then click Choose Library.

Photos will use this library until you open a different one.

Learn more

Move Photo Library Mac

If you encounter a permissions issue with your library, you may be able to resolve it by using the Photos library repair tool.

Iphoto Migrated Library

* If the volume isn't formatted APFS or Mac OS Extended (Journalled), or has been used for Time Machine backups but hasn't been erased, this tick box will either not be present, or won't be selectable after unlocking. Erase and reformat the drive for this option to be available.