You will need a Mac formatted external hard drive for flash drive. Connect it via USB.
To format your backup for Mac:
You need a large amount of storage (8GB or larger). If you are using a flash drive, start with a new one or copy everything to somewhere else. You are going to erase it.
Use Spotlight search feature (the magnifying glass in top right hand corner) and search for Disk Utility.
Select Disk Utility.
Click on flashdrive or hard drive.
Click on Erase button on the menu bar along the top of the box.
Click of Format, and you’ll get a pull down menu. Select ExFat – this allows you to save both Mac and PC projects.
Give your backup a Name.
Click Erase at the bottom of the box.
You will get a warning asking you if you are sure you want to erase the disk. Click Erase.
To save a project in iMovie 10:
Click on Project
>Copy Event to Library
>Select external hard drive
Do this often so you have a regular backup.
For a video tutorial of this process, click here.