Ubuntu – GRUB won’t boot persistent – “hd0,4 not found”

I'm trying to make a persistent USB flash drive of Ubuntu 17.04 with mkusb (dus). I've followed the directions on the mkusb website explicitly, and I've tried most variations I can think of, all with no joy. When I select the persistent USB in BIOS, grub boots from it; I select Ubuntu persistent; grub objects with hd0,4 not found. I've been searching online for a solution for several days now.

Some of the things I've tried:
– Creating the persistent drive on a second flash drive while booted from a live-USB flash drive;
– All combinations of BIOS legacy mode and secure boot enabled and disabled;
– msdos and gpt partition tables on the target drive;
– different flash drives;
– different amounts of persistent memory;
– etc.

Any suggestions will be greatly appreciated.

My system:
HP laptop with AMD64 quad core, 16GB memory, booting (usually) in UEFI secure mode. BIOS is the latest. The installed OS is Ubuntu 17.04.

Thanks!!
-Warren

Edit #1 to add:

On further investigation, it seems that grub loads from the flash drive as hd2, not hd0. This is probably because the two internal hard drives are discovered by the BIOS first and are recognized as hd0 and hd1. How to get grub on a persistent flash drive to recognize that it has been loaded from hd2 and not hd0 is presently above my pay grade.

Edit #2 to add:

@sudodus- thanks for the help. Some of the info requested:

The mkusb and dus components are all loaded from the ppa and are version 12.0.9. BIOS has legacy mode disabled and secure boot enabled. I tried editing the grub commands ("e") and changing hd0 to hd2. That works! The persistent flash drive boots as expected. (Wonder why grub doesn't just set root to the disk it was loaded from …?)

Output from the commands–

warren@warren-laptop:~$ sudo lsblk -fm
NAME   FSTYPE  LABEL              UUID                                 MOUNTPOINT                       NAME     SIZE OWNER GROUP MODE
sda                                                                                                     sda    953.9G root  disk  brw-rw----
├─sda1 vfat                       1168-7BED                            /boot/efi                        ├─sda1   256M root  disk  brw-rw----
├─sda2 ext4                       182d8f07-567e-4514-9f99-49b78767f195 /                                ├─sda2 921.6G root  disk  brw-rw----
└─sda3 swap                       8187044e-0939-4868-b81a-b5bc64bd9e7c [SWAP]                           └─sda3    32G root  disk  brw-rw----
sdb                                                                                                     sdb    698.7G root  disk  brw-rw----
└─sdb1 ext4    devel              27a0c109-82be-4ceb-b572-dedbeb412744 /home/warren/devel               └─sdb1 698.6G root  disk  brw-rw----
sdc                                                                                                     sdc     14.5G root  disk  brw-rw----
├─sdc1 ntfs    usbdata            134E2DAF76E6A9A4                     /media/warren/usbdata            ├─sdc1   3.2G root  disk  brw-rw----
├─sdc2                                                                                                  ├─sdc2     1M root  disk  brw-rw----
├─sdc3 vfat    usbboot            2EA6-44A9                                                             ├─sdc3   122M root  disk  brw-rw----
├─sdc4 iso9660 Ubuntu 17.04 amd64 2017-04-12-03-44-04-00               /media/warren/Ubuntu 17.04 amd64 ├─sdc4   1.5G root  disk  brw-rw----
└─sdc5 ext4    casper-rw          8a27c645-3e5c-4e7f-bf92-eb51dc0bd19f /media/warren/casper-rw          └─sdc5   9.6G root  disk  brw-rw----
warren@warren-laptop:~$ sudo parted -ls
Model: ATA SanDisk SD8SB8U1 (scsi)
Disk /dev/sda: 1024GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags: 

Number  Start   End     Size    File system     Name  Flags
 1      1049kB  269MB   268MB   fat32           efi   boot, esp
 2      269MB   990GB   990GB   ext4
 3      990GB   1024GB  34.4GB  linux-swap(v1)


Model: ATA ST750LX003-1AC15 (scsi)
Disk /dev/sdb: 750GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt
Disk Flags: 

Number  Start   End    Size   File system  Name   Flags
 1      16.8MB  750GB  750GB  ext4         devel


Model:  Patriot Memory (scsi)
Disk /dev/sdc: 15.5GB
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  130MB   128MB   fat32        primary  boot, esp
 4      130MB   1759MB  1629MB               primary
 5      1759MB  12.1GB  10.3GB  ext2         primary
 1      12.1GB  15.5GB  3436MB  ntfs         primary  msftdata


warren@warren-laptop:~$ df
Filesystem     1K-blocks      Used Available Use% Mounted on
udev             7776108         0   7776108   0% /dev
tmpfs            1559788     10204   1549584   1% /run
/dev/sda2      951092244 187079792 715676736  21% /
tmpfs            7798928        12   7798916   1% /dev/shm
tmpfs               5120         4      5116   1% /run/lock
tmpfs            7798928         0   7798928   0% /sys/fs/cgroup
/dev/sda1         258095      3496    254599   2% /boot/efi
/dev/sdb1      720011344 242201684 441165412  36% /home/warren/devel
tmpfs            1559784       168   1559616   1% /run/user/1000
/dev/sdc4        1571328   1571328         0 100% /media/warren/Ubuntu 17.04 amd64
/dev/sdc1        3355644     17388   3338256   1% /media/warren/usbdata
/dev/sdc5        9909328     36912   9352660   1% /media/warren/casper-rw
warren@warren-laptop:~$ 

Thanks!!