Sorry for long delay for this topic, but I have been heavily overloaded lately.
But today I read some extlinux source code, and I think my guess is right, syslinux way to make bootable disk is ancient version of msdos/linux MBR-copier. Seems that current extlinux MBR copier (on MBR.S assembler module) is using INT13 -BIOS interrupt to read disk parameters.
This is not working with USB-stick -> very possible system failure. But I'm not 100% sure if there is some alternative method for USB-stick, yet.
Edit: I have uninstalled RAID hd on my system, I'm not sure how syslinux sees it, probably not at all.
Edit2: TC is booted from another USB-stick=current disk -> probably not relevant if HD visible or not