diff --git a/src/app/sub.rs b/src/app/sub.rs
index 1b06a7a..8fe17e3 100644
--- a/src/app/sub.rs
+++ b/src/app/sub.rs
@@ -1,10 +1,11 @@
+mod mount_point;
+
 use crate::{
     args::Fraction,
     data_tree::{DataTree, DataTreeReflection},
     fs_tree_builder::FsTreeBuilder,
     get_size::GetSize,
     json_data::{BinaryVersion, JsonData, SchemaVersion, UnitAndTree},
-    mount_point::find_mountpoint,
     os_string_display::OsStringDisplay,
     reporter::ParallelReporter,
     runtime_error::RuntimeError,
@@ -12,9 +13,10 @@ use crate::{
     status_board::GLOBAL_STATUS_BOARD,
     visualizer::{BarAlignment, ColumnWidthDistribution, Direction, Visualizer},
 };
+use mount_point::find_mount_point;
 use serde::Serialize;
 use std::{fs, io::stdout, iter::once, num::NonZeroUsize, path::PathBuf};
-use sysinfo::{DiskKind, Disks};
+use sysinfo::{Disk, DiskKind, Disks};
 
 /// The sub program of the main application.
 pub struct Sub<Size, SizeGetter, Report>
@@ -74,24 +76,11 @@ where
         // If one of the files is on HDD, set thread number to 1
         let disks = Disks::new_with_refreshed_list();
 
-        if files
-            .iter()
-            .filter_map(|file| fs::canonicalize(file).ok())
-            .any(|path| {
-                let mount_points: Vec<_> = disks.iter().map(|disk| disk.mount_point()).collect();
-                if let Some(mount_point) = find_mountpoint(&path, &mount_points) {
-                    disks.iter().any(|disk| {
-                        matches!(disk.kind(), DiskKind::HDD) && disk.mount_point() == mount_point
-                    })
-                } else {
-                    false
-                }
-            })
-        {
+        if detect_hdd_in_files(&disks, &files, |disk| disk.kind()) {
             rayon::ThreadPoolBuilder::new()
                 .num_threads(1)
                 .build_global()
-                .unwrap();
+                .unwrap_or_else(|_| eprintln!("warning: This program is suboptimal with HDD"));
         }
 
         let mut iter = files
@@ -174,3 +163,24 @@ where
         Ok(())
     }
 }
+
+fn detect_hdd_in_files(
+    disks: &[Disk],
+    files: &[PathBuf],
+    get_disk_kind: impl Fn(&Disk) -> DiskKind,
+) -> bool {
+    files
+        .iter()
+        .filter_map(|file| fs::canonicalize(file).ok())
+        .any(|path| {
+            if let Some(mount_point) =
+                find_mount_point(&path, disks.iter().map(|disk| disk.mount_point()))
+            {
+                disks.iter().any(|disk| {
+                    get_disk_kind(disk) == DiskKind::HDD && disk.mount_point() == mount_point
+                })
+            } else {
+                false
+            }
+        })
+}
diff --git a/src/app/sub/mount_point.rs b/src/app/sub/mount_point.rs
new file mode 100644
index 0000000..397614e
--- /dev/null
+++ b/src/app/sub/mount_point.rs
@@ -0,0 +1,41 @@
+use std::{ffi::OsStr, path::Path};
+
+pub(super) fn find_mount_point<'a>(
+    path: &Path,
+    mount_points: impl IntoIterator<Item = &'a Path>,
+) -> Option<&'a Path> {
+    mount_points
+        .into_iter()
+        .filter(|mnt| path.starts_with(&*mnt.to_string_lossy()))
+        .max_by_key(|mnt| AsRef::<OsStr>::as_ref(mnt).len()) // Mount points can be nested in each other
+}
+
+#[cfg(test)]
+mod tests {
+    use super::find_mount_point;
+    use std::path::Path;
+
+    #[test]
+    fn test_mountpoint() {
+        let mount_points = [
+            Path::new("/"),
+            Path::new("/home"),
+            Path::new("/mnt/data"),
+            Path::new("/mnt/data/repo"),
+            Path::new("/mnt/repo"),
+        ];
+
+        for (path, mount_point) in &[
+            ("/etc/fstab", "/"),
+            ("/home/user", "/home"),
+            ("/mnt/data/repo/test", "/mnt/data/repo"),
+            ("/mnt/data/test/test", "/mnt/data/"),
+            ("/mnt/repo/test/test", "/mnt/repo/"),
+        ] {
+            assert_eq!(
+                find_mount_point(Path::new(path), mount_points).unwrap(),
+                Path::new(mount_point)
+            );
+        }
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 84ff2f2..fed11c1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -34,7 +34,6 @@ pub mod data_tree;
 pub mod fs_tree_builder;
 pub mod get_size;
 pub mod json_data;
-pub mod mount_point;
 pub mod os_string_display;
 pub mod reporter;
 pub mod size;
diff --git a/src/mount_point.rs b/src/mount_point.rs
deleted file mode 100644
index 1cf0ba9..0000000
--- a/src/mount_point.rs
+++ /dev/null
@@ -1,11 +0,0 @@
-use std::path::{Path, PathBuf};
-
-pub fn find_mountpoint(path: &Path, mount_points: &[&Path]) -> Option<PathBuf> {
-    let path = path.to_string_lossy();
-
-    mount_points
-        .iter()
-        .filter(|mnt| path.starts_with(&*mnt.to_string_lossy()))
-        .max_by_key(|mnt| mnt.to_string_lossy().len())
-        .map(|mnt| mnt.to_path_buf())
-}
diff --git a/tests/find_mountpoint.rs b/tests/find_mountpoint.rs
deleted file mode 100644
index 9377823..0000000
--- a/tests/find_mountpoint.rs
+++ /dev/null
@@ -1,26 +0,0 @@
-use parallel_disk_usage::mount_point::find_mountpoint;
-use std::path::Path;
-
-#[test]
-fn test_mountpoint() {
-    let mount_points = &[
-        Path::new("/"),
-        Path::new("/home"),
-        Path::new("/mnt/data"),
-        Path::new("/mnt/data/repo"),
-        Path::new("/mnt/repo"),
-    ];
-
-    for (path, mount_point) in &[
-        ("/etc/fstab", "/"),
-        ("/home/user", "/home"),
-        ("/mnt/data/repo/test", "/mnt/data/repo"),
-        ("/mnt/data/test/test", "/mnt/data/"),
-        ("/mnt/repo/test/test", "/mnt/repo/"),
-    ] {
-        assert_eq!(
-            find_mountpoint(Path::new(path), mount_points).unwrap(),
-            Path::new(mount_point)
-        );
-    }
-}