diff --git a/src/lib.rs b/src/lib.rs
index 97899e1..8722fad 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -251,7 +251,7 @@ impl Ext4 {
     /// Read bytes into `dst`, starting at `start_byte`.
     fn read_bytes(
         &self,
-        start_byte: u64,
+        mut start_byte: u64,
         mut dst: &mut [u8],
     ) -> Result<(), Ext4Error> {
         // The first 1024 bytes are reserved for non-filesystem
@@ -268,7 +268,7 @@ impl Ext4 {
             let block_index = self.0.journal.map_block_index(block_index);
             let offset_within_block = start_byte % block_size.to_u64();
 
-            let read_len = {
+            let read_len: usize = {
                 // OK to unwrap: `offset_within_block` is always less
                 // than the block size, and the block size always fits
                 // in a `u32`. We assume a `usize` is always at least as
@@ -280,14 +280,15 @@ impl Ext4 {
 
             let partial_dst = &mut dst[..read_len];
 
-            let start_byte =
+            let partial_start_byte =
                 block_index * block_size.to_u64() + offset_within_block;
             self.0
                 .reader
                 .borrow_mut()
-                .read(start_byte, partial_dst)
+                .read(partial_start_byte, partial_dst)
                 .map_err(Ext4Error::Io)?;
 
+            start_byte += util::u64_from_usize(read_len);
             dst = &mut dst[read_len..];
         }
 
diff --git a/src/util.rs b/src/util.rs
index d92fdb6..8fa503c 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -28,6 +28,26 @@ pub(crate) const fn usize_from_u32(val: u32) -> usize {
     }
 }
 
+/// Convert a `usize` to a `u64`.
+///
+/// TODO, but on platforms
+/// supported by this crate, this conversion is infallible.
+///
+/// # Panics
+///
+/// Panics if `val` does not fit in this platform's `u64`.
+#[inline]
+#[must_use]
+pub(crate) const fn u64_from_usize(val: usize) -> u64 {
+    assert!(size_of::<u64>() >= size_of::<usize>());
+
+    // Cannot use `usize::try_from` in a `const fn`.
+    #[expect(clippy::as_conversions)]
+    {
+        val as u64
+    }
+}
+
 /// Create a `u64` from two `u32` values.
 #[inline]
 #[must_use]