Index: kern/vfs_bio.c
===================================================================
--- kern/vfs_bio.c	(revision 189142)
+++ kern/vfs_bio.c	(working copy)
@@ -1170,6 +1170,7 @@
 
 	if (bp->b_iocmd == BIO_WRITE &&
 	    (bp->b_ioflags & BIO_ERROR) &&
+	    bp->b_error != ENXIO &&
 	    !(bp->b_flags & B_INVAL)) {
 		/*
 		 * Failed write, redirty.  Must clear BIO_ERROR to prevent
@@ -1177,6 +1178,9 @@
 		 * this case is not run and the next case is run to 
 		 * destroy the buffer.  B_INVAL can occur if the buffer
 		 * is outside the range supported by the underlying device.
+		 * If the error is that the device went away (ENXIO), we
+		 * shouldn't redirty the buffer either, but discard the
+		 * data too.
 		 */
 		bp->b_ioflags &= ~BIO_ERROR;
 		bdirty(bp);
Index: kern/vfs_mount.c
===================================================================
--- kern/vfs_mount.c	(revision 189142)
+++ kern/vfs_mount.c	(working copy)
@@ -1293,8 +1293,13 @@
 		error = VFS_UNMOUNT(mp, flags, td);
 	}
 	vn_finished_write(mp);
-	if (error) {
-		/* Undo cdir/rdir and rootvnode changes made above. */
+	/*
+	 * If we failed to flush the dirty blocks for this mount point,
+	 * undo all the cdir/rdir and rootvnode changes we made above.
+	 * Unless we failed to do so because the device is reporting that
+	 * it doesn't exist anymore.
+	 */
+	if (error && error != ENXIO) {
 		if ((flags & MNT_FORCE) &&
 		    VFS_ROOT(mp, LK_EXCLUSIVE, &fsrootvp, td) == 0) {
 			if (mp->mnt_vnodecovered != NULL)