“iMovie Library” is already in use by “user” on "user's computer".

I spent most of the day yesterday on this issue (including an hour+ on the phone with Apple Support) before discovering a resolution using ChatGPT along with lots of trial and error.

First, try copying the external iMovie Library to your local drive. WARNING: iCloud folders appear to be local (AKA on your Mac), but they are not. If you copy to such a folder, you’ll get a message indicating: “This document cannot be opened from its current location. Copy the document to a local storage device, then try again.” I did not understand this problem or error message until much later.

Once your iMovie Library is in a truly local folder, it should open. However,…

My specific situation was likely due to using an external drive (or USB stick) that was in the NTFS (AKA Microsoft) format. You can use Mac’s “Disk Utility” to check your external disk’s format (look for the “Type” indication). This issue might also apply to other Microsoft disk formats (e.g., FAT, etc.) although I can’t say for sure.

One way to check whether you have the same issue as I did (after verifying that your external disk or USB stick is in an NTFS format) is to test whether the disk/stick is writable. If it’s not, this indicates a larger “permissions” problem. You’ll notice that you can’t change these permissions as you usually would from the file’s “Get Info” screen. Keep going…

The next step is to upgrade the standard Mac/Apple NTFS drivers. According to ChatGPT, “By default, macOS can read from NTFS-formatted drives, but it has limited write support. This limitation can sometimes lead to permissions-related problems, especially when accessing the drive from multiple Macs or different user accounts.” Exactly!

ChatGPT went on to say “You can install third-party NTFS drivers that provide full read and write support for NTFS drives on macOS. One popular option is Paragon NTFS for Mac. After installing the driver, you should be able to read and write to the external drive without any permissions issues.”

After I installed the Paragon app using the free trial version AND rebooting, I was able to read and write to the external drive as the suspect permissions issue disappeared. After that, I could open the iMovie Library directly from the external disk.

A key point to set expectations: Installing Paragon NTFS for Mac, like with a few other specialized applications, requires you to modify your security settings in the “Recovery” environment. Search for how to reboot and launch Mac’s “Startup Security Utility”. It might be the hardest part of the whole process.

Happy iMovie editing!