Why does Apple not recommend storing your iMovie library on an external drive?
"Apple doesn’t recommend storing iMovie libraries on external storage devices like SD cards and USB flash drives, or drives shared on a network."

'cause the quality of external flash cards are poor.  I have like eight flash drives bought in the last eight years and two of them have failed.  External cards use a double layer of circuits where as internal SSDs use one. 

Library of Congress has a question and answer email facility.  I sent them a line about flash drives.  I found out that:


  LoC does not have data on the reliability of usb memory sticks.
  LoC recommends the use of rotating magnetic media for long term digital storage.


Here is what Library of Congress says about long term storage photos.  I assume that vids would be the same. 

This is controversial around these parts, but I recommend you get your photos out of the Apple ecosystem and store them as one photo per file on multiple external storage devices. For long term storage, don't rely on usb memory sticks nor external ssd drives.  LoC does not have data on the reliability of usb memory sticks.  Apple seems to be saying do not store photos on external ssd drives.  One device should be off site.  Using a cloud service is fine.  Remember a lot of cloud services are sync services.  That is should the file be deleted on the local device, the photo will be deleted in the cloud.  For this reason,  I never install the cloud service software on my computer.  I use only file upload.  For those who complain, my assessment is that the apple Eco-system doesn't match Library of Congress's requirements for digital archiving.

informative video:
http://www.digitalpreservation.gov/multimedia/videos/personalarchiving-photos.html

printed version
http://www.digitalpreservation.gov/personalarchiving/photos.html

General information on digital preservation:
http://www.digitalpreservation.gov/personalarchiving/

R