I realized I can use my USB2-A meter to test the output from the USB2 ports. Problematic PSU causes it to have 5.3V and the better one has 5.1V. Hopefully it helps anyone else having problems with a stronger PSU.
Its a bit irritating, as /etc/usbmount/usbmount.conf contains the line FILESYSTEMS="vfat ext2 ext3 ext4 hfsplus ntfs exfat" but the way MOUNTOPTIONS are set, only vfat and ntfs are actually supported, as all others will fail to mount.
Hm, that may actually be a bug. Let me re-test and maybe we can correct this.
Hmm. I followed your advice and copied the video-splashscreen over to /home/pi/splashscreens and changed the line in /etc/splashscreen.list accordingly. Strangely this doesn't work, the splashscreens never appears.
I even double- and triple-checked the paths and tried different folders, but the splashscreen won't show up.
Just got it to work. After copying it over to /opt/retropie/supplementary/splashscreen/ to the default splashscreens and selecting it it works like a charm!