Hi Sevamma, thanks for your reply, but I tried that a long time ago when I made my first attempt at upgrading to Mavericks and 10.0.9 - it had no effect on performance or any of the other problems I was having.
However, I have found a partial fix to my new problems upgrading external drives to 10.1 and Mavericks ...I noticed that a few of my external drive's Sharing & Permissions were set to "read only" and the "Ignore ownership on this volume" was unchecked. I've had problems with FCPX and these settings in the past, so I restored the external drive to it's original state from a backup, set all the permissions to "read & write", checked the ignore ownership box, and then attempted to update the Projects and Events through 10.1 again... it worked this time without an error! It's odd, because the permissions have changed on their own from time to time. Anyone know why?
The only issues I've encountered so far is that each updated project's timeline needs to pre-render again in the project's timeline (10.1 seams to be overstepping the project's old render files and wants to create new ones), which takes a while... it makes a pre-rendering pass... does some stabilizing/ optical flow analysis... does some more pre-rendering... most of the project's orange render bar clears, but some still remains... scrubbing/ playing the playhead over a few of those leftover orange areas prompts FCPX to render them, but... I'm still left with a few orange areas that won't complete their optical flow anaylsis. Those areas drop frames during playback/ stutter in the exported file... so I'm left with no option but to tweak them in some way so that it forces FCPX to re-pre-render the clip in the timeline. A simple roll of the clip by a single frame in either direction is enough to force it, but I wish I didn't have to do any rendering/ tweaking in order to work with my old projects.
My advice... backup all your drives, set all external disk permissions to read & write, check the ignore ownership box and then cross your fingers before updating. Also, save your old projects and events folders when FCPX asks you just to be safe. If things appear to have gone smoothly during the update, go through all your projects to make sure everything's working as it should before you make your next backup (all of my external drives have a cloned backup, so a stitch in time saves nine before overwriting your backup).
Beta tester reporting back for duty, lol.