I used unetbootin to make a bootable USB key (a BT4 persistant live cd)
But it wors only on a few machines. (only 1 out of 6 i tried) Most of the time boot sequence get stuck on a blinking prompt cursor.
Attempting Boot From Floppy Drive (A:)
Attempting Boot From USB Device
_ <-- stuck...
Then boot on HDD
Is it do to the usb key or machines that do not load mbr the same way??
Edit: the stick is formated with MBR have 2 partitions. The first is FAT32 and active, second is ext3
PCs i have tested can boot on USB (at leat you can select a USB device for Boot)