I see this problem pretty frequent while google. Rendering my xps15z pretty useless as a long to be in linux.
Boot from 8GB PNY usb thumb drive. use unetbootin. Have tried persistent and non persistent. have changed syslinux to point at casper, and found solution suggestions telling to extract casper from iso. did but then realized casper is already on usb. have also tried with and without "live-media-path"
below is the way im currently configured.
Please help!!label unetbootindefault
menu label Default
append initrd=casper/initrd.gz file=/cdrom/preseed/custom.seed boot=casper live-media-path=casper text splash vga=791--
EDIT:: I was able to get BT5 R1 to boot on the 15z by connecting the boot USB via the USB/eSATA Port and NOT one of the two usb3.0 ports..