diff --git a/init.sh b/init.sh
index 8bcb9d3..186befa 100755
--- a/init.sh
+++ b/init.sh
@@ -213,6 +213,78 @@ libreportalDerivePaths
 # passed, or when the scan finds nothing else — a prompt with one possible
 # answer is not a question. Self-contained (findmnt only): scripts/ is not
 # necessarily loadable this early.
+# Say something when another drive is attached but not usable yet. Deliberately
+# only a notice: mounting or formatting someone's disk is not a thing an
+# installer should do without being asked, and `storage add` does it properly
+# later with the empty-directory admission rule and the fitness checks.
+_initMentionUnmounted()
+{
+    command -v lsblk >/dev/null 2>&1 || return 0
+
+    # "Has no mount point" is not the same as "is free". The disk holding the
+    # root filesystem has no mount point of its own — its PARTITION does — so a
+    # naive check offers the user the disk they booted from. Mark anything
+    # mounted or claimed as in use, then propagate that up the parent chain.
+    declare -A _used=() _child=() _type=() _size=() _fs=() _parent=()
+    local line kv val name type size mnt fstype pkname
+    # -P, not -r: an unmounted device has an EMPTY mount point, and in raw output
+    # that is just a run of spaces, so `read` collapses it and every column after
+    # it shifts left. An LVM member then reads as a mount point and is offered as
+    # free space. Key="value" pairs cannot slide.
+    while IFS= read -r line; do
+        [[ -z "$line" ]] && continue
+        name=""; type=""; size=""; mnt=""; fstype=""; pkname=""
+        for kv in NAME TYPE SIZE MOUNTPOINT FSTYPE PKNAME; do
+            val="${line#*${kv}=\"}"
+            [[ "$val" == "$line" ]] && continue
+            val="${val%%\"*}"
+            case "$kv" in
+                NAME) name="$val" ;; TYPE) type="$val" ;; SIZE) size="$val" ;;
+                MOUNTPOINT) mnt="$val" ;; FSTYPE) fstype="$val" ;; PKNAME) pkname="$val" ;;
+            esac
+        done
+        [[ -z "$name" ]] && continue
+        _type[$name]="$type"; _size[$name]="$size"; _fs[$name]="$fstype"
+        _parent[$name]="$pkname"
+        [[ -n "$pkname" ]] && _child[$pkname]=1
+        [[ -n "$mnt" ]] && _used[$name]=1
+        case "$fstype" in swap|LVM2_member|crypto_LUKS|linux_raid_member) _used[$name]=1 ;; esac
+    done < <(lsblk -Pno NAME,TYPE,SIZE,MOUNTPOINT,FSTYPE,PKNAME 2>/dev/null)
+
+    # Anything holding something in use is itself in use, all the way up.
+    local n p
+    for n in "${!_used[@]}"; do
+        p="${_parent[$n]:-}"
+        while [[ -n "$p" ]]; do _used[$p]=1; p="${_parent[$p]:-}"; done
+    done
+
+    local -a spare=()
+    for n in "${!_type[@]}"; do
+        [[ -n "${_used[$n]:-}" ]] && continue
+        case "$n" in loop*|sr*|zram*|ram*|fd*) continue ;; esac
+        # A disk with partitions is described by those partitions; offering both
+        # would list the same space twice.
+        [[ "${_type[$n]}" == "disk" && -n "${_child[$n]:-}" ]] && continue
+        [[ "${_type[$n]}" == "disk" || "${_type[$n]}" == "part" ]] || continue
+        # Skip the small ones — EFI and boot stubs are not app-data drives.
+        case "${_size[$n]}" in *G|*T) ;; *) continue ;; esac
+        spare+=("/dev/$n  ${_size[$n]}${_fs[$n]:+  ${_fs[$n]}}${_fs[$n]:+}")
+        [[ -z "${_fs[$n]}" ]] && spare[-1]="${spare[-1]}  (no filesystem yet)"
+    done
+
+    (( ${#spare[@]} )) || return 0
+
+    echo ""
+    isNotice "Another drive is attached but not mounted, so it cannot be used yet:"
+    local s
+    for s in "${spare[@]}"; do printf '    %s\n' "$s"; done
+    echo ""
+    echo "  Mount it and run this installer again to install onto it, or add it"
+    echo "  afterwards with:  libreportal storage add /path/to/mountpoint"
+    echo ""
+    return 0
+}
+
 initPickRoots()
 {
     [[ "$init_unattended_mode" == true ]] && return 0
@@ -250,7 +322,16 @@ initPickRoots()
         labels+=("$(printf '%-24s %-8s %s free' "$target" "${size:-?}" "${avail:-?}")")
     done < <(findmnt -Pno TARGET,SOURCE,FSTYPE,SIZE,AVAIL 2>/dev/null)
 
-    (( ${#paths[@]} )) || return 0
+    # Nothing mounted to offer. That is not the same as "no other disk": a
+    # brand-new drive is usually unmounted, sometimes unformatted, and this
+    # prompt is the only place the installer ever mentions that app data can
+    # live somewhere else. Returning in silence taught those users the feature
+    # does not exist.
+    if (( ${#paths[@]} == 0 )); then
+        _initMentionUnmounted
+        return 0
+    fi
+
 
     local root_size root_avail
     root_size=$(findmnt -no SIZE  --target / 2>/dev/null | tail -1)
diff --git a/scripts/dev/lp-installer-disks-test b/scripts/dev/lp-installer-disks-test
new file mode 100755
index 0000000..3dc79f3
--- /dev/null
+++ b/scripts/dev/lp-installer-disks-test
@@ -0,0 +1,113 @@
+#!/bin/bash
+# Does the installer mention a spare drive — and only a genuinely spare one?
+#
+#   scripts/dev/lp-installer-disks-test
+#
+# initPickRoots can only offer MOUNTED filesystems, so on a box whose second
+# drive is new (unmounted, often unformatted) it returned in silence and the
+# prompt never appeared at all. That taught exactly the users who have a spare
+# disk that the feature does not exist.
+#
+# The notice that fills that gap has to be careful in two specific ways, and
+# both are cases below:
+#
+#   1. "Has no mount point" is not "is free". The disk holding the root
+#      filesystem has no mount point of its own — its partition does — so the
+#      obvious check offers the user the disk they booted from.
+#   2. lsblk's raw output renders an empty mount point as a run of spaces, which
+#      `read` collapses, shifting every later column left. An LVM member then
+#      parses as a mount point and gets offered as free space. Hence -P.
+#
+# Runs the REAL _initMentionUnmounted extracted from init.sh against a stubbed
+# lsblk, so editing the heuristic is what makes this fail.
+
+REPO="$(cd "$(dirname "$0")/../.." && pwd)"
+fail=0
+chk(){ if [[ "$2" == "$3" ]]; then echo "  ok   $1"; else echo "  FAIL $1: got '$2' want '$3'"; fail=1; fi; }
+
+FN=$(awk '/^_initMentionUnmounted\(\)/,/^\}/' "$REPO/init.sh")
+[[ -n "$FN" ]] || { echo "  FAIL could not extract _initMentionUnmounted from init.sh"; exit 1; }
+
+# Fixtures are written as lsblk -P rows. The stub HONOURS the output flag it is
+# called with: asked for -r it re-renders them raw, collapsing the empty fields
+# the way lsblk really does. Without that the stub answers -P to everything and
+# the test cannot tell the two parsers apart — which is the bug being guarded
+# against, so a stub that ignores the flag guards nothing.
+STUB_SRC=$(mktemp)
+{
+  echo 'lsblk() {'
+  echo '    case " $* " in'
+  echo '        *" -P"*) cat "$ROWS_FILE"; return 0 ;;'
+  echo '    esac'
+  echo '    sed -E '"'"'s/[A-Z]+="([^"]*)"/\1/g; s/  +/ /g; s/^ +| +$//g'"'"' "$ROWS_FILE"'
+  echo '}'
+} > "$STUB_SRC"
+trap 'rm -f "$STUB_SRC"' EXIT
+
+runrows() {
+    local tmp rf out
+    tmp=$(mktemp); rf=$(mktemp)
+    cat > "$rf"
+    {
+        echo 'isNotice(){ echo "NOTICE: $*"; }'
+        cat "$STUB_SRC"
+        echo "$FN"
+        echo '_initMentionUnmounted'
+    } > "$tmp"
+    out=$(ROWS_FILE="$rf" bash "$tmp" 2>/dev/null)
+    rm -f "$tmp" "$rf"
+    printf '%s\n' "$out"
+}
+
+ROOT_ROWS='NAME="nvme0n1" TYPE="disk" SIZE="931.5G" MOUNTPOINT="" FSTYPE="" PKNAME=""
+NAME="nvme0n1p1" TYPE="part" SIZE="1G" MOUNTPOINT="/boot/efi" FSTYPE="vfat" PKNAME="nvme0n1"
+NAME="nvme0n1p2" TYPE="part" SIZE="930G" MOUNTPOINT="" FSTYPE="crypto_LUKS" PKNAME="nvme0n1"
+NAME="dm-0" TYPE="crypt" SIZE="930G" MOUNTPOINT="/" FSTYPE="ext4" PKNAME="nvme0n1p2"'
+
+echo "--- a normal single-disk box (root on LUKS over nvme0n1p2) ---"
+out=$(runrows <<< "$ROOT_ROWS")
+chk "offers nothing" "$(grep -c NOTICE <<< "$out")" "0"
+
+echo "--- a spare unformatted disk ---"
+out=$(runrows <<< "$ROOT_ROWS
+NAME=\"sdb\" TYPE=\"disk\" SIZE=\"3.6T\" MOUNTPOINT=\"\" FSTYPE=\"\" PKNAME=\"\"")
+chk "mentions it"      "$(grep -c NOTICE <<< "$out")"          "1"
+chk "names the device" "$(grep -c '/dev/sdb' <<< "$out")"      "1"
+chk "says it is blank" "$(grep -c 'no filesystem yet' <<< "$out")" "1"
+
+echo "--- a spare formatted partition ---"
+out=$(runrows <<< "$ROOT_ROWS
+NAME=\"sdb\" TYPE=\"disk\" SIZE=\"3.6T\" MOUNTPOINT=\"\" FSTYPE=\"\" PKNAME=\"\"
+NAME=\"sdb1\" TYPE=\"part\" SIZE=\"3.6T\" MOUNTPOINT=\"\" FSTYPE=\"ext4\" PKNAME=\"sdb\"")
+chk "names the partition"        "$(grep -c '/dev/sdb1' <<< "$out")"      "1"
+chk "not the whole disk as well" "$(grep -cE '/dev/sdb  ' <<< "$out")"    "0"
+
+echo "--- an LVM member is not free space (the column-shift case) ---"
+out=$(runrows <<< "$ROOT_ROWS
+NAME=\"sdb\" TYPE=\"disk\" SIZE=\"2T\" MOUNTPOINT=\"\" FSTYPE=\"\" PKNAME=\"\"
+NAME=\"sdb1\" TYPE=\"part\" SIZE=\"2T\" MOUNTPOINT=\"\" FSTYPE=\"LVM2_member\" PKNAME=\"sdb\"")
+chk "offers nothing" "$(grep -c NOTICE <<< "$out")" "0"
+
+echo "--- a mounted spare is already usable, so the picker handles it ---"
+out=$(runrows <<< "$ROOT_ROWS
+NAME=\"sdb\" TYPE=\"disk\" SIZE=\"2T\" MOUNTPOINT=\"\" FSTYPE=\"\" PKNAME=\"\"
+NAME=\"sdb1\" TYPE=\"part\" SIZE=\"2T\" MOUNTPOINT=\"/mnt/data\" FSTYPE=\"ext4\" PKNAME=\"sdb\"")
+chk "offers nothing" "$(grep -c NOTICE <<< "$out")" "0"
+
+echo "--- a partition whose CHILD is mounted (nothing on the part itself) ---"
+# sdb1 has no mount point and no member fstype of its own; only dm-1 below it is
+# mounted. Without walking the parent chain, sdb1 looks like free space.
+out=$(runrows <<< "$ROOT_ROWS
+NAME=\"sdb\" TYPE=\"disk\" SIZE=\"2T\" MOUNTPOINT=\"\" FSTYPE=\"\" PKNAME=\"\"
+NAME=\"sdb1\" TYPE=\"part\" SIZE=\"2T\" MOUNTPOINT=\"\" FSTYPE=\"\" PKNAME=\"sdb\"
+NAME=\"dm-1\" TYPE=\"crypt\" SIZE=\"2T\" MOUNTPOINT=\"/data\" FSTYPE=\"ext4\" PKNAME=\"sdb1\"")
+chk "offers nothing" "$(grep -c NOTICE <<< "$out")" "0"
+
+echo "--- small stubs are not app-data drives ---"
+out=$(runrows <<< "$ROOT_ROWS
+NAME=\"sdb\" TYPE=\"disk\" SIZE=\"512M\" MOUNTPOINT=\"\" FSTYPE=\"\" PKNAME=\"\"")
+chk "offers nothing" "$(grep -c NOTICE <<< "$out")" "0"
+
+echo ""
+if (( fail )); then echo "FAILED"; exit 1; fi
+echo "All installer disk-notice checks passed."