[syslinux] Can USBstikA boot USBstkB ?!

[eas lab]

Yes this is the SAME question, that I can't get an answer from a month ago!

I have the files, from a USBstik [now stolen] which I previously could
edit at boot time, to boot any of my IDE or SATA partitions.

Now, with a laptop, I want to boot the same IDE or SATA via a USB-adaptor
to IDE/SATA. And also I want to boot a Toshiba 500GB USB disk.

The obvious question is: why [if] the old system could boot any IDE/SATA
partition, it can not boot a second USBdevice. The laptop is running 64b.
This is what the good old 32bit USBstik's files look like:------------
-rwxr-xr-x 1 root root     708 Oct 20  2015 message.txt
-rwxr-xr-x 1 root root     906 Oct 20  2015 sylinux.cfg
-rwxr-xr-x 1 root root    1138 Oct 20  2015 syslinux.cfg
-rwxr-xr-x 1 root root  123172 Oct 20  2015 vesamenu.c32
-rwxr-xr-x 1 root root 4940304 Oct 20  2015 vmlinuz
------- message.txt file reads:-----------------------
Welcome to the Slackware Linux custom USB boot stick!

By default, this stick boots a root Linux partition on /dev/sda3 when
you hit ENTER.  If you'd like to boot some other partition, use a
command like this on the prompt below:

    mount root=/dev/hda15 ro vga=791.

Where "/dev/sda1" is the partition you want to boot, and "ro" specifies
that the partition should be initially mounted as read-only.  If you
wish to mount the partition read-write, use "rw" instead.  To set the
video console mode, use the vga= parameter (press F1 to see a table).
You may also add any other kernel parameters you might need depending
on your hardware, and which drivers are included in your kernel.
------------------------   end of file: message.txt -----------------

I suspect that <up-calls> eg. like old INT13, can't be made to access
additional USBports, like they could be to FIXED-LOCATION disk
partitions, eg. /dev/hdb5 ??