So I'm trying to make a live persistent USB drive running ubuntu. I've succeded so far but the problem is that I have to put UEFI boot mode to make it work. However, if I do that, my system doesn't boot into windows so I have to change it again into Legacy.

Needless to say I would like to have the usb boot with legacy mode (or windows to boot with UEFI).

I'm using mkusb to make the bootable drive and I've read What are the mkusb options "msdos", "upefi" and "d-n-i"? but it's still not clear to me which one will work. I tired "use default" and "upefi" but they don't work.

I have a message that to use "grub.img" but if I choose "yes" I get a fail message:

mount: /tmp/dus.t5oXMA9yGr: special device /dev/sdb3 does not exist.
sed: can't read /tmp/dus.t5oXMA9yGr/boot/grub/grub.cfg: No such file or directory
failed :-(
umount: /dev/sdb3: not mounted.
Syncing the target device ...
item 90
Wait 5 seconds and a little more ...
parted -s "/dev/sdb" print
Model: SanDisk Ultra Fit (scsi)
Disk /dev/sdb: 62,1GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags: 

Number  Start   End     Size    File system  Name     Flags
 2      1049kB  2097kB  1049kB               primary  bios_grub
 3      2097kB  258MB   256MB   fat32        primary  boot, esp
 4      258MB   2231MB  1973MB               primary
 5      2231MB  62,1GB  59,9GB  ext2         primary
 1      62,1GB  62,1GB  1237kB               primary  msftdata

MODEL            NAME   FSTYPE  LABEL                    MOUNTPOINT  SIZE
Ultra Fit        sdb                                                57,9G
                 |-sdb1                                              1,2M
                 |-sdb2                                                1M
                 |-sdb3 vfat    usbboot                              244M
                 |-sdb4 iso9660 Ubuntu 18.04.1 LTS amd64             1,9G
                 `-sdb5 ext4    casper-rw                           55,8G
  - sed: tweak 3 grub.cfg
 Failed :-( Check carefully what happened! 
item 100
Cleanup after dus-persistent finished :-)
Cleanup after dus-persistent finished :-)
Total time used [by dus-persistent] = 299 s; 00:04:59

How can I make both systems boot with the same BIOS configuration?

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