diff --git a/bdk/libs/fatfs/ff.c b/bdk/libs/fatfs/ff.c
index 81e951da..c6b05e3c 100644
--- a/bdk/libs/fatfs/ff.c
+++ b/bdk/libs/fatfs/ff.c
@@ -3933,15 +3933,12 @@ FRESULT f_read_fast (
 {
 	FRESULT res;
 	FATFS *fs;
-	UINT csize_bytes;
-	DWORD clst;
-	UINT count = 0;
+	UINT csize_bytes, count;
+	DWORD clst, work_bytes;
 	FSIZE_t work_sector = 0;
 	FSIZE_t sector_base = 0;
 	BYTE *wbuff = (BYTE*)buff;
 
-	// TODO support sector reading inside a cluster
-
 	res = validate(&fp->obj, &fs);			/* Check validity of the file object */
 	if (res != FR_OK || (res = (FRESULT)fp->err) != FR_OK) {
 		EFSPRINTF("FOV");
@@ -3951,9 +3948,15 @@ FRESULT f_read_fast (
 	if (!(fp->flag & FA_READ)) LEAVE_FF(fs, FR_DENIED); /* Check access mode */
 	FSIZE_t remain = fp->obj.objsize - fp->fptr;
 	if (btr > remain) btr = (UINT)remain;		/* Truncate btr by remaining bytes */
+	if (!btr)
+		goto out;
 
 	csize_bytes = fs->csize * SS(fs);
 
+	//!TODO: support sector reading inside a cluster
+	DWORD csect = (UINT)((fp->fptr / SS(fs)) & (fs->csize - 1));	/* Sector offset in the cluster */
+	if (csect) { EFSPRINTF("ICSR"); ABORT(fs, FR_INT_ERR); }
+
 	if (!fp->fptr) {	/* On the top of the file? */
 		clst = fp->obj.sclust;	/* Follow from the origin */
 	} else {
@@ -3966,9 +3969,10 @@ FRESULT f_read_fast (
 	fp->clust = clst;	/* Set working cluster */
 
 	sector_base = clst2sect(fs, fp->clust);
-	count += fs->csize;
-	btr -= csize_bytes;
-	fp->fptr += csize_bytes;
+	work_bytes = MIN(btr, csize_bytes);
+	count = work_bytes / SS(fs);
+	fp->fptr += work_bytes;
+	btr -= work_bytes;
 
 	while (btr) {
 		clst = clmt_clust(fp, fp->fptr);	/* Get cluster# from the CLMT */
@@ -3979,26 +3983,27 @@ FRESULT f_read_fast (
 		fp->clust = clst;
 
 		work_sector = clst2sect(fs, fp->clust);
-		if ((work_sector - sector_base) == count) count += fs->csize;
+		work_bytes = MIN(btr, csize_bytes);
+		if ((work_sector - sector_base) == count) count += work_bytes / SS(fs);
 		else {
 			if (disk_read(fs->pdrv, wbuff, sector_base, count) != RES_OK) ABORT(fs, FR_DISK_ERR);
 			wbuff += count * SS(fs);
 
 			sector_base = work_sector;
-			count = fs->csize;
+			count = work_bytes / SS(fs);
 		}
 
-		fp->fptr += MIN(btr, csize_bytes);
-		btr -= MIN(btr, csize_bytes);
-
-		// TODO: what about if data is smaller than cluster?
-		// Must read-write back that cluster.
-
-		if (!btr) {	/* Final cluster/sectors read. */
-			if (disk_read(fs->pdrv, wbuff, sector_base, count) != RES_OK) ABORT(fs, FR_DISK_ERR);
-		}
+		fp->fptr += work_bytes;
+		btr -= work_bytes;
 	}
 
+	if (!btr) {	/* Final cluster/sectors read. */
+		if (work_bytes % SS(fs))
+			count++; /* Read an extra block. Expects buffer being big enough. */
+		if (disk_read(fs->pdrv, wbuff, sector_base, count) != RES_OK) ABORT(fs, FR_DISK_ERR);
+	}
+
+out:
 	LEAVE_FF(fs, FR_OK);
 }
 #endif
@@ -4158,15 +4163,12 @@ FRESULT f_write_fast (
 {
 	FRESULT res;
 	FATFS *fs;
-	UINT csize_bytes;
-	DWORD clst;
-	UINT count = 0;
+	UINT csize_bytes, count;
+	DWORD clst, work_bytes;
 	FSIZE_t work_sector = 0;
 	FSIZE_t sector_base = 0;
 	const BYTE *wbuff = (const BYTE*)buff;
 
-	// TODO support sector writing inside a cluster
-
 	res = validate(&fp->obj, &fs);			/* Check validity of the file object */
 	if (res != FR_OK || (res = (FRESULT)fp->err) != FR_OK) {
 		EFSPRINTF("FOV");
@@ -4174,6 +4176,9 @@ FRESULT f_write_fast (
 	}
 
 	if (!(fp->flag & FA_WRITE)) LEAVE_FF(fs, FR_DENIED);	/* Check access mode */
+	if (!btw)
+		goto out;
+
 	/* Check fptr wrap-around (file size cannot reach 4 GiB at FAT volume) */
 	if ((!FF_FS_EXFAT || fs->fs_type != FS_EXFAT) && (DWORD)(fp->fptr + btw) < (DWORD)fp->fptr) {
 		btw = (UINT)(0xFFFFFFFF - (DWORD)fp->fptr);
@@ -4181,22 +4186,26 @@ FRESULT f_write_fast (
 
 	csize_bytes = fs->csize * SS(fs);
 
+	// TODO support sector writing inside a cluster
+	DWORD csect = (UINT)((fp->fptr / SS(fs)) & (fs->csize - 1));	/* Sector offset in the cluster */
+	if (csect) { EFSPRINTF("ICSR"); ABORT(fs, FR_INT_ERR); }
+
 	if (!fp->fptr) {	/* On the top of the file? */
 		clst = fp->obj.sclust;	/* Follow from the origin */
 	} else {
 		if (fp->cltbl) clst = clmt_clust(fp, fp->fptr);	/* Get cluster# from the CLMT */
 		else { EFSPRINTF("CLTBL"); ABORT(fs, FR_CLTBL_NO_INIT); }
 	}
-
 	if (clst < 2) { EFSPRINTF("CCHK"); ABORT(fs, FR_INT_ERR); }
 	else if (clst == 0xFFFFFFFF) { EFSPRINTF("DERR"); ABORT(fs, FR_DISK_ERR); }
 
 	fp->clust = clst;	/* Set working cluster */
 
 	sector_base = clst2sect(fs, fp->clust);
-	count += fs->csize;
-	btw -= csize_bytes;
-	fp->fptr += csize_bytes;
+	work_bytes = MIN(btw, csize_bytes);
+	count = work_bytes / SS(fs);
+	fp->fptr += work_bytes;
+	btw -= work_bytes;
 
 	while (btw) {
 		clst = clmt_clust(fp, fp->fptr);	/* Get cluster# from the CLMT */
@@ -4207,28 +4216,30 @@ FRESULT f_write_fast (
 		fp->clust = clst;
 
 		work_sector = clst2sect(fs, fp->clust);
-		if ((work_sector - sector_base) == count) count += fs->csize;
+		work_bytes = MIN(btw, csize_bytes);
+		if ((work_sector - sector_base) == count) count += work_bytes / SS(fs);
 		else {
 			if (disk_write(fs->pdrv, wbuff, sector_base, count) != RES_OK) ABORT(fs, FR_DISK_ERR);
 			wbuff += count * SS(fs);
 
 			sector_base = work_sector;
-			count = fs->csize;
+			count = work_bytes / SS(fs);
 		}
 
-		fp->fptr += MIN(btw, csize_bytes);
-		btw -= MIN(btw, csize_bytes);
+		fp->fptr += work_bytes;
+		btw -= work_bytes;
+	}
 
-		// what about if data is smaller than cluster?
-		// Probably must read-write back that cluster.
-		if (!btw) {	/* Final cluster/sectors write. */
-			if (disk_write(fs->pdrv, wbuff, sector_base, count) != RES_OK) ABORT(fs, FR_DISK_ERR);
-			fp->flag &= (BYTE)~FA_DIRTY;
-		}
+	if (!btw) {	/* Final cluster/sectors write. */
+		if (work_bytes % SS(fs))
+			count++; /* Write an extra block. Cluster is always > storage block. */
+		if (disk_write(fs->pdrv, wbuff, sector_base, count) != RES_OK) ABORT(fs, FR_DISK_ERR);
+		fp->flag &= (BYTE)~FA_DIRTY;
 	}
 
 	fp->flag |= FA_MODIFIED;	/* Set file change flag */
 
+out:
 	LEAVE_FF(fs, FR_OK);
 }
 #endif
@@ -6754,7 +6765,6 @@ int f_puts (
 {
 	putbuff pb;
 
-
 	if (str == (void *)0) return EOF; /* String is NULL */
 
 	putc_init(&pb, fp);
@@ -6782,7 +6792,6 @@ int f_printf (
 	DWORD v;
 	TCHAR c, d, str[32], *p;
 
-
 	if (fmt == (void *)0) return EOF; /* String is NULL */
 
 	putc_init(&pb, fp);
diff --git a/bdk/libs/fatfs/ff.h b/bdk/libs/fatfs/ff.h
index 3a6c4238..720fee6c 100644
--- a/bdk/libs/fatfs/ff.h
+++ b/bdk/libs/fatfs/ff.h
@@ -264,8 +264,10 @@ FRESULT f_open (FIL* fp, const TCHAR* path, BYTE mode);				/* Open or create a f
 FRESULT f_close (FIL* fp);											/* Close an open file object */
 FRESULT f_read (FIL* fp, void* buff, UINT btr, UINT* br);			/* Read data from the file */
 FRESULT f_write (FIL* fp, const void* buff, UINT btw, UINT* bw);	/* Write data to the file */
+#if FF_FASTFS /* buff needs to be block aligned. Intercluster data access is not supported. */
 FRESULT f_read_fast (FIL* fp, const void* buff, UINT btr);			/* Fast read data from the file */
 FRESULT f_write_fast (FIL* fp, const void* buff, UINT btw);         /* Fast write data to the file */
+#endif
 FRESULT f_lseek (FIL* fp, FSIZE_t ofs);								/* Move file pointer of the file object */
 FRESULT f_truncate (FIL* fp);										/* Truncate the file */
 FRESULT f_sync (FIL* fp);											/* Flush cached data of the writing file */
@@ -287,7 +289,9 @@ FRESULT f_getfree (const TCHAR* path, DWORD* nclst, FATFS** fatfs);	/* Get numbe
 FRESULT f_getlabel (const TCHAR* path, TCHAR* label, DWORD* vsn);	/* Get volume label */
 FRESULT f_setlabel (const TCHAR* label);							/* Set volume label */
 FRESULT f_forward (FIL* fp, UINT(*func)(const BYTE*,UINT), UINT btf, UINT* bf);	/* Forward data to the stream */
+#if FF_FASTFS
 DWORD  *f_expand_cltbl (FIL* fp, UINT tblsz, FSIZE_t ofs);			/* Expand file and populate cluster table */
+#endif
 FRESULT f_expand (FIL* fp, FSIZE_t fsz, BYTE opt);					/* Allocate a contiguous block to the file */
 FRESULT f_mount (FATFS* fs, const TCHAR* path, BYTE opt);			/* Mount/Unmount a logical drive */
 FRESULT f_mkfs (const TCHAR* path, BYTE opt, DWORD au, void* work, UINT len);	/* Create a FAT volume */