Q: HELP! Need to move photo libraries off iMac!

How are you managing the photos? Photos app, iPhoto or something else?

Photos stores the library in your 'Pictures' folder by default.

http://www.howtogeek.com/214569/how-to-move-your-apple-photos-library-to-another -location/

You can probably just move that & then edit the preferences to point to the new location (quit the app first & a backup would be a good idea too).

If you Mac is currently throwing up 'out of space errors' I suggest you consider freeing up some space first. Copying 700GB will take a long time & you need to avoid hang to restart the task. Personally I trust Carbon Copy Cloner to move large quantities of data around as it can verify it copied correctly. Finder can fail to copy & has no resume or verify feature.

http://bombich.com/

first we need details

what Version of the OS and or Photos? What are your iCloud settings on the iMac? How much free space do you have on the iMac?

LN