"The document ___ could not be opened. Permission denied (The file is already in use by.....)"

I suppose that ___ in question is the name of the library that you were using, and want to continue working on, right?

1) Quit FCP if it is running

2) In the Finder, locate the library in question, control-click, Show Package Contents

3) Press Command-Shift-period to have Finder show hidden files

4) Drag the two files and one folder whose names start with .lock to the trash

5) Close the contents, double-click the library to open in FCP

As Tom said before, if you can get the media from that drive (and I certainly expect that you can), then it is probably best to pick a recent backup of the library - from ~/Movies/Final Cut Backups - and relink the media.

As it happens, your 15GB of library could really be in great part due to render files and optimized media.

So maybe you can try copying off the original media to your system drive, space permitting.

Do Show Package Contents, and copy of the contents of the Original Media subfolders.

Do as Tom and Luis have suggested and copy the media out of the broken library, open a backup of the library and then relink to your media. Do this on your new APFS drive.

It's possible the library has been corrupted by the unexpected drive disconnect and the versioning error upon re-opening is not accurate. It's my understanding that exFAT is not a journaled file system so the risk of data loss is greater than with HFS+ or APFS. Probably why Apple specifies them for the ProApps. Format storage devices for Final Cut Pro, Motion, and Compressor - Apple Support

Many people keep multiple versions of FCP in case there's a need to return to an older project, so keeping different FCP versions is common and recommended.

With the library selected in the Finder, command-click the window title to reveal the path, and select the drive. Apparently, it is called "Lacie #1". With the drive selected, press Command-I just as you did to show the Info window for the library.

Your drive is formatted as ExFAT. That is not a good thing for use with FCP, especially with libraries (media files might be ok, but even then I'd not use ExFAT).

Besides, the "You can only read" seems to be a problem.

I'd try copying this library to an APFS or HFS Extended formatted drive.

If you can copy the media out of the copied library bundle. There may not be enough space. I don’t know if ExFAT supports Mac hard links. You could also just move the media out of the library. If you open a backup you can relink the media. The media itself on an ExFAT is usually not a problem; it’s the library and the media being in the library bundle that doesn’t work correctly.

Is your library on the drive or just the media? How is the drive formatted?

Post screenshots of the library and the drive permissions.

I assume you’ve already rebooted.

It’s the drive format. It cannot be used with FCP. It will damage the library. Of you cannot copy the contents or clone the drive, you might try a drive recovery service. You should have a data backup of the library in your Movies folder. You might be able to open that. If you can open that you can relink the media if you can get it off the LaCie.

Powering off the drive or disconnecting while it’s in use will likely damage the drive directory and cause the problems you’re seeing.

Select the drive on the desktop and use Cmd-I. It will be at the bottom of Info.

You might try using a cloning app like Carbon Copy Cloner to clone the drive to an APFS formatted SSD.

Can you post a screenshot of the message? Where does the message appear? Is your library on the drive or just the media? How is the drive formatted?

I don’t know if there’s anything you can do. The drive is ExFAT, which you cannot use with FCP for libraries. That is why it has no permissions settings and they’re fixed.

Just speculating, but maybe you had another older copy of FCP somewhere.

If this happens, you can do Final Cut Pro->About Final Cut Pro to confirm which version is actually running.

Nothing else there? Custom access is strange.

Where are the drive permissions?

I’m not surprised it’s not working correctly of you keep using ExFAT.

Is the media inside the library or outside it?

Were you able to open a backup?