Move photo libraries from iMac macOS Monterey to hard drives

First: To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted.

Move your Photos library to save space on your Mac           

Let us know if you need more explanation…

Are you planning to use an SSD drive for faster performance? Then you may want to remember, to connect your SSD drive occasionally to your Mac to refresh the storage, if it does not have its own power source.

SSD drive need to refresh the data occasionally, if they have been without power supply for a long time.

Are your very old photo libraries perhaps still iPhoto or Aperture libraries? If you are still having iPhoto or Aperture Libraries you may want to create Photos Libraries from them. Your current system version, MacOS 12 Monterey is the last version of macOS where we can migrate the older libraries to Photos as a library and preserve the albums and events and keywords. The more recent system versions can only import the media (images and videos), but not the structure of the library, see: Migrating an Aperture Library to Photos a… - Apple Community. Monterey is our last chance to preserve the structure of an Aperture or iPhoto library.