iMovie Library to external hardrive
If i delete my imovie library from my imac because I already have it on an external harddrive (I just copied it over), will my movie currently being edited be affected?
The better way is to move, instead of copy, your original library to the external drive. Do an Edit/Copy, Edit/Paste into the external drive. Once you confirm that everything is working properly on the external drive, delete your original library on your internal drive. Before transferring the library, make sure that your external drive is formatted Mac OS extended (journaled) or APFS. If you need to reformat the drive be aware that reformatting will delete all data on the drive. So first back up any data that you want to keep.
Also, it is prudent to keep a double backup of your iMovie library like you presently have. If you delete your original library you will have only one library, on the external drive, and no backup unless you keep a Time Machine Backup.
To address your original question: Your iMovie project currently being edited will not be affected if you delete the original library and revert to the backup library. Provided, however, that before you delete the original library you re-copy it at that point in time so that all of the edits in your projects are current in the backup library that you copied over. So, if you copy over your original library on Day 1, and then do some new edits in your original library on Day 3, and then delete the original library, the .new edits would not appear in the copied over library. You would need first to copy the original library over again on Day 3 before deleting.
-- Rich