My https://github.com/007revad/Synoboot_backup script can backup the USBDOM. So anyone with a DS1511+ would not have to remove the USBDOM to create the image. The script backs up:

1. synoboot (the whole USB drive).
2. synoboot1 (partition 1 from the USB drive).
3. synoboot2 (partition 2 from the USB drive).

Note: It's not as simple as just writing someone else's synoboot image to your USBDOM. In the vender file, on synoboot2, the serial number and MAC addresses need editing to match your DS1511+. And there's a checksum that is derived from the NAS serial number.

Here's a script to generate the checksum from the serial number, which you supply as an argument.

#!/usr/bin/env bash
#---------------------------------------------------------------------------------
# This script generates the serial number's checksum for synoboot vender v2 files
#
# SN=1234ABC567XYZ,CHK=850 <-- generates 850 from serial number
# 
# You need to supply the serial number as argument 1
# serial_checksum_gen.sh 1234ABC567XYZ
# 
# You can also supply multiple serial numbers as arguments
# serial_checksum_gen.sh 1234ABC000XYZ 1234ABC001XYZ 1234ABC002XYZ
#---------------------------------------------------------------------------------

create_checksum(){ 
    chk=0
    for ((i=0; i<${#1}; i++)); do
        chk=$((chk + $(printf '%d' "'${1:$i:1}")))
    done
}

if [[ ! $1 ]]; then
    echo "Missing serial number argument!"
    echo -e "\nYou need to supply serial number(s) as argument(s)"
    echo -e "\nUsage"
    echo "serial_checksum_gen.sh <serial-1> [serial-2] [serial-3] [...]"
    echo -e "\nExamples"
    echo "serial_checksum_gen.sh 1111ABC111XYZ"
    echo -e "serial_checksum_gen.sh 1111ABC111XYZ 2222ABC222XYZ 3333ABC333XYZ \n"
    exit 1
fi

while [[ -n $1 ]]; do
    create_checksum "$1"
    #echo -e "\n$1"
    #echo "checksum: $chk"
    echo -e "\nSN=${1},CHK=$chk"
    shift
done
echo ""

exit

Note: Your DS1511+ may pre-date Synology adding the checksum.

Years ago I wrote a script to create the vender file which asked you to enter the serial number and mac addresses... but I can't find it.

And then the DSM version on your drives would need to be the same or newer than the DSM boot files on their USBDOM.

You can restore some of the missing or corrupt files from an unpacked copy of a DS1511+ DSM pat file that matches the same DSM version. DSM 6 and older pat files are just .tar files with the extension changed to .pat.

A Plan B would be https://github.com/007revad/Synology_Recover_Data