(Clip Import Issue) Must close and reopen Final Cut Pro X

Just to be clear, are you talking about:

a) dragging a clip from outside FCP X (e.g. from the Finder) directly to a FCP X timeline?

or

b) dragging a clip from the browser to the timeline?

I assume you mean a) since the title of your post uses the word "Import".

If so, does it work if you actually use File->Import to import files?

FWIW, even though it is supported, dragging a file directly to the timeline has drawbacks, IMHO: you do not have access to the Import Window, so all the options are taking from defaults - including to which event the clip goes, whether or not optimized/proxy is created, and whether the file is copied or left in place. I much prefer to confirm those settings visually in the Import window; besides, I rarely want to use the full length of the clip in the timeline.

Same issue bu FCP would not restart.

Imported video from Photos and from iPhone X, FCP showed a box for the clip but the video was not visible, when I tried dragging to Timeline it would not connect to the timeline.

Tried dragging from Photos directly into Timeline and that did to work either

What Alchroma said.

There are ways to remedy this, or perhaps just wait until tomorrow... there's a good chance 10.4 will come out, with support for HEVC. Why tomorrow? Well, Apple has announced the iMac Pro will be available tomorrow, and 10.4 is supposed to shine on it, and has also been announced to ship this year.

Cannot reproduce this and have not seen it reported elsewhere. Can you give details of the media and what you're dragging into. Maybe post screenshots.

Sounds like your issue is the HEVC codec that the iPhone X uses.

FCP X does not currently support this.

Al