diff --git a/ports/zephyr-cp/prj.conf b/ports/zephyr-cp/prj.conf
index be3c56ef755..39b4206ff88 100644
--- a/ports/zephyr-cp/prj.conf
+++ b/ports/zephyr-cp/prj.conf
@@ -33,6 +33,7 @@ CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT=n
 
 CONFIG_USBD_MSC_CLASS=y
 CONFIG_USBD_MSC_LUNS_PER_INSTANCE=1
+CONFIG_USBD_MSC_SCSI_BUFFER_SIZE=4096
 
 CONFIG_HWINFO=y
 CONFIG_REBOOT=y
diff --git a/ports/zephyr-cp/supervisor/flash.c b/ports/zephyr-cp/supervisor/flash.c
index 9f0db1f3031..ad1158431db 100644
--- a/ports/zephyr-cp/supervisor/flash.c
+++ b/ports/zephyr-cp/supervisor/flash.c
@@ -616,7 +616,9 @@ static bool _flash_write_block(const uint8_t *data, uint32_t block) {
             return write_flash(address, data, FILESYSTEM_BLOCK_SIZE);
         }
         if (_current_page_address != NO_PAGE_LOADED) {
-            supervisor_flash_flush();
+            // Flush only the old page. supervisor_flash_flush() would also
+            // clear the dirty flag, which littlefs sets before writing.
+            _flush_keep_cache(true);
         }
         if (flash_cache_table == NULL && !allocate_ram_cache()) {
             erase_page(_scratch_page_address);
diff --git a/ports/zephyr-cp/supervisor/usb.c b/ports/zephyr-cp/supervisor/usb.c
index b6e7d3c9d07..becbdee41b6 100644
--- a/ports/zephyr-cp/supervisor/usb.c
+++ b/ports/zephyr-cp/supervisor/usb.c
@@ -199,7 +199,8 @@ int _zephyr_disk_ioctl(struct disk_info *disk, uint8_t cmd, void *buff) {
             disk_ioctl(root, GET_SECTOR_SIZE, buff);
             return 0;
         case DISK_IOCTL_CTRL_SYNC:
-            disk_ioctl(root, CTRL_SYNC, buff);
+            // Zephyr's MSC class sends this after every WRITE10. Don't flush
+            // here; the background flush writes the cached page.
             autoreload_resume(AUTORELOAD_SUSPEND_USB);
 
             // This write is complete; initiate an autoreload if this was a file data or metadata write,
diff --git a/ports/zephyr-cp/tests/test_flash.py b/ports/zephyr-cp/tests/test_flash.py
index 7882d1f31a8..8e81491ba24 100644
--- a/ports/zephyr-cp/tests/test_flash.py
+++ b/ports/zephyr-cp/tests/test_flash.py
@@ -194,6 +194,96 @@ def test_flash_256k_overwrite(circuitpython, board):
     assert content == "second version"
 
 
+PAGE_CHANGE_CODE = """\
+import board
+import storage
+import time
+# The display's auto refresh keeps the supervisor tick running, which would
+# flush the cache whether or not the filesystem is marked dirty.
+board.DISPLAY.auto_refresh = False
+storage.remount("/", readonly=False)
+# Program the sectors first. Writes to erased flash skip the page cache.
+with open("/data.bin", "wb") as f:
+    f.write(bytes(16384))
+# Overwrite one sector, then one three pages later, so the last write moves
+# the flash cache to a new page. Don't flush or close the file.
+f = open("/data.bin", "r+b")
+f.write(b"A" * 512)
+f.seek(12288)
+f.write(b"Z" * 512)
+# The background flush runs after 1000 supervisor ticks. native_sim's 100 Hz
+# kernel tick makes that about 10 seconds.
+time.sleep(15)
+print("slept")
+while True:
+    time.sleep(1)
+"""
+
+
+@pytest.mark.circuitpy_drive({"code.py": PAGE_CHANGE_CODE})
+@pytest.mark.fat_filesystem_only
+def test_flash_background_flush_after_page_change(circuitpython):
+    """Test that the background flush writes the page cached by a page change.
+
+    The file is never flushed or closed, so only the background flush can
+    write the last sector before the simulator is stopped.
+    """
+    circuitpython.serial.wait_for("slept")
+    circuitpython.shutdown()
+
+    assert b"Z" * 512 in circuitpython.flash_file.read_bytes()
+
+
+LFS_PAGE_CHANGE_CODE = """\
+import board
+import os
+import storage
+import time
+# The display's auto refresh keeps the supervisor tick running, which would
+# flush the cache whether or not the filesystem is marked dirty.
+board.DISPLAY.auto_refresh = False
+storage.remount("/", readonly=False)
+# littlefs writes new data to unused blocks, and writes to erased flash skip
+# the page cache. Fill the free space and free it again so they don't.
+f = open("/burn.bin", "wb")
+chunk = bytes(8192)
+try:
+    while True:
+        f.write(chunk)
+except OSError:
+    pass
+f.close()
+os.remove("/burn.bin")
+# Creating the file syncs, so the cache holds the metadata page. The first
+# block of the file is on another page, so writing it changes pages. Don't
+# flush or close the file.
+f = open("/z.bin", "wb")
+f.write(b"Z" * 512)
+# The background flush runs after 1000 supervisor ticks. native_sim's 100 Hz
+# kernel tick makes that about 10 seconds.
+time.sleep(15)
+print("slept")
+while True:
+    time.sleep(1)
+"""
+
+
+@pytest.mark.circuitpy_drive({"code.py": LFS_PAGE_CHANGE_CODE})
+@pytest.mark.parametrize("board", ["native_native_sim_lfs"], indirect=True)
+def test_flash_background_flush_after_page_change_lfs(circuitpython):
+    """Test that the background flush writes the page cached by a littlefs write.
+
+    littlefs marks the filesystem dirty before writing, so the page change
+    must not clear the dirty flag. The file is never flushed or closed, so
+    only the background flush can write the block before the simulator is
+    stopped.
+    """
+    circuitpython.serial.wait_for("slept")
+    circuitpython.shutdown()
+
+    assert b"Z" * 512 in circuitpython.flash_file.read_bytes()
+
+
 SCRATCH_PAGE_CODE = """\
 import gc
 import storage
diff --git a/supervisor/shared/flash.c b/supervisor/shared/flash.c
index 61a6f03c444..ef7c32aadd9 100644
--- a/supervisor/shared/flash.c
+++ b/supervisor/shared/flash.c
@@ -178,27 +178,31 @@ static mp_uint_t flash_write_blocks(mp_obj_t self_in, const uint8_t *src, uint32
         // can't write MBR, but pretend we did
         return 0;
     } else {
-        supervisor_flash_mark_dirty();
         block_num -= PART1_START_BLOCK;
         #if CIRCUITPY_SAVES_PARTITION_SIZE > 0
         mp_vfs_blockdev_t *self = (mp_vfs_blockdev_t *)self_in;
         block_num += self->offset / self->block_size;
         #endif
-        return supervisor_flash_write_blocks(src, block_num, num_blocks);
+        mp_uint_t result = supervisor_flash_write_blocks(src, block_num, num_blocks);
+        // Mark dirty after the write, so a flush running in another thread
+        // can't clear the flag between marking and caching the block.
+        supervisor_flash_mark_dirty();
+        return result;
     }
 }
 
 void PLACE_IN_ITCM(supervisor_flash_flush)(void) {
+    // Clear the flag before flushing. A write that lands during the flush
+    // marks the filesystem dirty again and turns the tick back on.
+    if (filesystem_dirty) {
+        filesystem_dirty = false;
+        supervisor_disable_tick();
+    }
     #if INTERNAL_FLASH_FILESYSTEM
     port_internal_flash_flush();
     #else
     supervisor_external_flash_flush();
     #endif
-    // Turn off ticks now that our filesystem has been flushed.
-    if (filesystem_dirty) {
-        supervisor_disable_tick();
-    }
-    filesystem_dirty = false;
 }
 
 static mp_obj_t supervisor_flash_obj_readblocks(mp_obj_t self, mp_obj_t block_num, mp_obj_t buf) {