Backup a small file on your xSiiD or similar while still allowing normal function (e.g. URL on scan) for unused space. File can be protected from deletion and also optionally protected from reading using password. Note you should still encrypt the file if it is sensitive! Please be careful as you can't recover password if forgotten!

write-nfc-file.sh

#!/bin/bash
read -sp"Password (8 hex chars raw or string): " p
echo
if ! grep -q "^[0-9A-Fa-f]\{8\}$" <<< "$p"
then
    echo "You provided a text password."
    echo "Hashing with: echo -n \"PASSWORD\" | sha256sum"
    echo "The password will be the first 8 chars of the hash."
    p=$(echo -n "$p" | sha256sum)
    p=${p:0:8}
    echo "Hashed password: $p"
    echo "============"
    echo
fi

l=$(cat "$1"|wc -c)
x=$((226-l/4))

if [[ $((l%4)) != 0 ]]
then
    x=$((x-1))
fi

if [[ "$x" -lt 4 ]]
then
    echo "Too much data!"
    exit 1
fi

echo "To set a new password if this is factory set xSiiD:"
echo "hf mfu wrbl -b 229 -d $p -k 444E4752"
echo
echo "To set a new password if this is another tag w/o password:"
echo "hf mfu wrbl -b 229 -d $p"
echo "============"
echo

echo "To prevent writes to zip area:"
echo "hf mfu wrbl -b 227 -d 000000$(printf "%x\n" $x) -k $p"
echo "============"
echo

echo "To protect the data from reads as well without password:"
echo "hf mfu wrbl -b 228 -d 80000000 -k $p"
echo "Note: Requires the previous \"prevent writes\" command."
echo "============"
echo

echo "To remove data protections (except password):"
echo "hf mfu wrbl -b 228 -d 00000000 -k $p"
echo "hf mfu wrbl -b 227 -d 000000E2 -k $p"
echo "============"
echo

echo "To write new data:"
xxd -c 4 -p "$1" | while read -r line
do
    while [[ ${#line} -lt 8 ]]
    do
        line="${line}00"
    done
    echo "hf mfu wrbl -b $((x++)) -d $line -k $p"
done

echo "==========="
echo "READING DATA"
echo "To read from proxmark:"
echo "hf mfu dump -f filename -k $p"
echo
echo "To read using NFC tools shell on android:"
echo "1B$(tr '[:lower:]' '[:upper:]' <<< "$p"),3A00FF"
echo
echo "To find these commands more easily from memory, note that:"
echo "1B is the PWD_AUTH command from the datasheet, which has the"
echo "8 char hex password after it."
echo "3A00FF is the READ_FAST command set to dump all possible data"
echo "(00 start FF end)"
echo
echo "After you dump it, you'll need to find the start of your file"
echo "and manually remove everything before it. On android you also"
echo "need to convert the tag data from hex back to binary. You can"
echo "use Termux on Android to help with this, namely edit the hex"
echo "with vim to remove the zeros before the data at the end of"
echo "the tag data, and then use \"cat dump | xxd -r -p > my_file\""