Imac Late 2012-21"- having problem with iMovie every time i start it. take one hour to validate library

First, make sure that you have sufficient free space on your internal disk for iMovie to function efficiently. Many folks recommend 20-25GB of free space, although iMovie will run O.K. with 10Gb. Also if you store your library on an external drive make sure that there is sufficient space on the drive for the library to open.

If the above is all O.K, try these two diagnostic procedures:

Sometimes iMovie can run slowly if the iMovie library is very large, or they may be some corruption in the library, so you can try a new library. Open iMovie in a new library by holding down the Option key while opening and selecting to create a new library in the box that appears. See if iMovie works faster.

If still no luck, try deleting preferences. Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

Although you have reset the PRAM, try resetting the System Management Controller (but no longer available for the new M1 computers):

https://support.apple.com/en-us/HT201295

-- Rich

Here are a couple of help links that discuss how to work with multiple libraries and how to manage projects and events.

https://help.apple.com/imovie/mac/10.1/#/mov3fa25bae7

https://help.apple.com/imovie/mac/10.1/#/mov74d7d370c

There is no best way to organize iMovie files. It depends upon your workflow and your needs. Everyone is different in that regard. I sort Events by name in alphabetical order. I sort clips by ascending date. I don't need anything beyond that. As for libraries, I create a new one for every year. I also have libraries dedicated solely to testing out projects using different editing techniques, and I keep the projects to refer back to. I keep two external drives formatted Mac OS Extended (Journaled) for library backups. I share out all important projects to movies that I store in a Movies Archive folder on a backup drive. That's because in future years the apps may become obsolete and you won't be able to open the projects. I store all source media in a finder folder organized by year, rather than have the iMovie library be the only place where it exists. For safety you never want your media to be stored in an app, whether iMovie or Photos, as the only place where it exists. Apps can corrupt or be outdated over years, or become inoperable on one's current operating system. I also store a lot of media in the Photos app, that has far more comprehensive tools for organiizing than does iMovie, such as albums and keyworks, and search features. If you want to do a serious job of organizing then the Photos app is where to be rather than iMovie. iMovie does have organizing tools, but not nearly as comprehensive as the Photos app. Fortunately, you can access the Photos app from within iMovie.

1. No, external drives are plenty fast enough. If it is any slower you likely won't notice the difference. You would still be using the same CPU and graphics card. I have all of my several iMovie libraries stored on external drives.
2. iMovie libraries must be stored on drives formatted Mac OS Extended (Journaled) or APFS in order to avoid data loss. ExFat or anything else won't work.

-- Rich

You are welcome.

It is perfectly feasible, although not necessary from a performance standpoint, to make a separate library for each project. That would leave you with a lot of libraries.

If you use media that might not be located in the Original Media folder of the particular library that contains the project, as a precaution after you complete the project select the library name in the media sidebar and do a File/Consolidate Library Media. That will make sure that you have a duplicate copy in the library of all media that is used in the project. Otherwise, if you move your library or change the location of your source files, iMovie may not be able to find the media you would get a "missing file" notification when you try to play the project.

-- Rich

Photos in the finder on your internal drive will be duplicated when you import them into iMovie. So you will have duplicate files on your internal drive that will take up twice the space. If you have space issues, you could always delete your source photos off of your internal drive after importing them into iMovie. A better solution would be to move the source photos onto an external drive for storage. It is safer to avoid having your source photos stored solely in iMovie. Not only are they harder to access and organize there for other purposes, but if your iMovie library gets corrupted or this particular iMovie version in later years becomes obsolete, you could lose your source files.

Another way to save space on your internal drive is to store your iMovie library on an external drive that is formatted Mac OS Extended (Journaled) or APFS.

As far as I know, iMovie does not use referenced clips, although there are File/Consolidate Project Media and File/Consolidate Library Media functions that would imply that media located outside of the iMovie library can be used in projects. Such media would necessarily have to be referenced, but I have not been able to confirm this. Everything that I drag in from a folder located on an external drive goes into the iMovie Library and thus is duplicated. However, the full size original source files also remain on the external drive.

-- Rich

I can't give you a definitive answer since I don't use Final Cut Pro. From what I have read, the Seagate gets overall good reviews, the main knock being failures. It is a 5,400 rpm drive rather than 7,200 rpm, so one might think that it is slower, but I haven't seen any complaints about speed. People use it for games. So, since you already have the drive, try it and see for yourself. Your Final Cut Pro application, that makes the movies, would likely be located on your internal drive, with the library on the external drive. I suspect that your Seagate would be fine for storing your movies.

I have two WD drives on which I store duplicates of everything, so if one drive fails I have the other.

-- Rich

However, SSD drives are faster than HHD drives, so if you are really concerned about speed, go with an SSD drive. Here's a link to an article that compares SSD drives witih HHD drives. It even mentions the Seagate 5.400 rpm drive that you have.

https://www.intel.com/content/www/us/en/products/docs/memory-storage/solid-state-drives/ssd-vs-hdd.html

-- Rich

An external spin drive should work fine., as well as an SSD. I have an external spin drive that I store my iMovie libraries on, and I don't have any speed problems.

-- Rich