I was initially having the same problem ("no partition found") with a 200GB SanDisk card that had been used without issue before. The *only* thing that worked was using guiformat, but setting the sector size to 64 KiB, *not* 4 KiB, or any other size for that matter. (This was the case, btw, on both the X3 and a Clip Zip.)
Meanwhile, I'm having two other issues, which do not seem to have been addressed (yes, I searched the thread, and yes, I'm using the latest available firmware from xvortex's site). One is a problem playing Opus files that it seems like RB tried patching in its last release, but still lingers on some of my newer encoded files. For example, on a ~180kbps file, playback happens for 00:00-00:01, then skip to 00:04 and continues from there. On some very low bitrate files (e.g. encoded from audiobooks) the issue is even worse - it'll skip to 00:02, and then all the way to 00:06 or 00:07. This appears to be happening with my most recently encoded files using opustools 0.1.10 (the build that ships with foobar2000's encoder pack), but not with all of them. I have not isolated the differences between the files that work and files that don't as yet, but will update the thread once I do. This happens on my Clip Zip as well, so my guess is this is not specific to Xvortex's port, and I'll be crossposting at Rockbox.org.
(ED: This appears to happen with embedded artwork, and not even large artwork at that - just 50-60kb.
The forum thread at rockbox.org is here, and I'll be filing a bug report after uh, getting some sleep...)
The second issue is more severe, though, and specific to the XDuoo X3. Sometimes, when scrolling down using the left arrow key, the player will randomly select the currently-highlighted file/folder/option, as if I pressed the big round play key. Sometimes, pressing the key acts as the pause button, as well. This does not occur in the stock FW. Not sure if there's some sensitivity setting or something that I can adjust, but it's absolutely maddening. Has anybody else been experiencing this? I tried searching the thread but nothing came up, and the search term to use is not obvious.