user3486: So I wanted to do a mashup video for personal usage, meaning scenes cut from downloaded commercial movies or Youtube. This is typically relatively highly compressed h264-material.
The workflow that I attempted, but feel is not optimal, is to:

Remux losslessly to .mov containers and stereo-only audio where applicable
On 2x speed, watch through in QuickTime Player X. Whenever I want to use something, I use the Trim function, Duplicate the trimmed movie, save the duplicate, and then undo the changes in the original and continue.
In FCPX, I import the clips and put them in a project.

The problems in my workflow is, that it would be great with a tool that is more optimal to scan large amount of video (e.g. a full length movie) and mark sections to become clips. Secondly, FCPX frequently crashes when I try to use the clips natively. I have converted some of them to Optimized Media (Prores 422).
Is there a better tool, or a workflow combining tools, that makes it particularly easy to do these types of video mashups?

user3675: Not on FCPX, but on Avid.  However, this is what I do and it may help:

Download at the maximum possible resolution you can download at.
Use AVIDemux as my rough-cut tool to mark 'A' and 'B' and save as clips with meaningful names.  Leave a few seconds on either side. 
Import clips into Avid

Sometimes, I have to convert the container to 'MP4' or resample frame rates and AVIDemux handles these well.

user2836: Why are you not simply using the video editing package to scrub for clips?  My preference has always been to do things in the editing software directly.  I'm a Premiere guy not a FCPX guy, but is there any method to form a virtual clip based on given start and end points in FCPX?  How powerful is your system if it is crashing trying to scrub H264 files?  It sounds like there are some bigger issues going on with your hardware or software.  Either that or FCP has really truly fallen off the wagon with X.

user4167: Another option worth exploring would be to use Cineform Studio (http://gopro.com/software-app/cineform-studio) from GoPro.
It allows you to mark clips within a larger movie file, and to individually export the clips as separate .mov files, which can then be imported into your favourite editor.
While Cineform Studio is developed by GoPro, I've found that it works fine on H.264 DSLR video  footage. I can't confirm whether it will work for a full-length movie, however.

3486: <p>So I wanted to do a mashup video for personal usage, meaning scenes cut from downloaded commercial movies or Youtube. This is typically relatively highly compressed h264-material.</p>

<p>The workflow that I attempted, but feel is not optimal, is to:</p>

<ol>
<li>Remux losslessly to .mov containers and stereo-only audio where applicable</li>
<li>On 2x speed, watch through in QuickTime Player X. Whenever I want to use something, I use the Trim function, Duplicate the trimmed movie, save the duplicate, and then undo the changes in the original and continue.</li>
<li>In FCPX, I import the clips and put them in a project.</li>
</ol>

<p>The problems in my workflow is, that it would be great with a tool that is more optimal to scan large amount of video (e.g. a full length movie) and mark sections to become clips. Secondly, FCPX frequently crashes when I try to use the clips natively. I have converted some of them to Optimized Media (Prores 422).</p>

<p>Is there a better tool, or a workflow combining tools, that makes it particularly easy to do these types of video mashups?</p>
 3675: <p>Not on FCPX, but on Avid.  However, this is what I do and it may help:</p>

<ol>
<li>Download at the maximum possible resolution you can download at.</li>
<li>Use AVIDemux as my rough-cut tool to mark 'A' and 'B' and save as clips with meaningful names.  Leave a few seconds on either side. </li>
<li>Import clips into Avid</li>
</ol>

<p>Sometimes, I have to convert the container to 'MP4' or resample frame rates and AVIDemux handles these well.</p>
 2836: <p>Why are you not simply using the video editing package to scrub for clips?  My preference has always been to do things in the editing software directly.  I'm a Premiere guy not a FCPX guy, but is there any method to form a virtual clip based on given start and end points in FCPX?  How powerful is your system if it is crashing trying to scrub H264 files?  It sounds like there are some bigger issues going on with your hardware or software.  Either that or FCP has really truly fallen off the wagon with X.</p>
 4167: <p>Another option worth exploring would be to use Cineform Studio (<a href="http://gopro.com/software-app/cineform-studio" rel="nofollow">http://gopro.com/software-app/cineform-studio</a>) from GoPro.</p>

<p>It allows you to mark clips within a larger movie file, and to individually export the clips as separate .mov files, which can then be imported into your favourite editor.</p>

<p>While Cineform Studio is developed by GoPro, I've found that it works fine on H.264 DSLR video  footage. I can't confirm whether it will work for a full-length movie, however.</p>
