Fix disk selection for installer prompts

scripts/install-btrfs.sh

@@ -85,6 +85,7 @@ if (( ! NON_INTERACTIVE )); then
       disk_choices+=("$d" "")
     done
     DISK=$(prompt_select "disk" "Select target disk (will be wiped)" "${disk_choices[@]}")
+    DISK=${DISK%% *} # strip size suffix from selection
   fi
 
   target_mode=$(prompt_select "mode" "Partitioning mode" \
@@ -129,6 +130,11 @@ mapper_root=""
 
 echo "[1/8] Partitioning/selection"
 if [[ -z "$ROOT_PART" ]]; then
+  if [[ ! -b "$DISK" ]]; then
+    echo "Error: target disk ${DISK} not found. Available disks:"
+    ls_disks
+    exit 1
+  fi
   printf "label: gpt\n,550M,U,boot\n,,L,cryptroot\n" | sfdisk "${DISK}"
   part_boot="${DISK}1"
   part_root="${DISK}2"