diff --git a/lib/ac-api/src/main/java/com/webcodepro/applecommander/storage/os/rdos/RdosFormatDisk.java b/lib/ac-api/src/main/java/com/webcodepro/applecommander/storage/os/rdos/RdosFormatDisk.java
index a1bd45a..8c9a03b 100644
--- a/lib/ac-api/src/main/java/com/webcodepro/applecommander/storage/os/rdos/RdosFormatDisk.java
+++ b/lib/ac-api/src/main/java/com/webcodepro/applecommander/storage/os/rdos/RdosFormatDisk.java
@@ -67,15 +67,38 @@ public class RdosFormatDisk extends FormattedDisk {
 	 */
 	public static final int ENTRY_LENGTH = 0x20;
 	/**
-	 * Specifies the number of blocks on the disk.  
+	 * Specifies the number of tracks on the disk.  
 	 * RDOS apparantly only worked on 5.25" disks.
 	 */
-	public static final int BLOCKS_ON_DISK = 455;
+	public static final int TRACKS_ON_DISK = 35;
+	/**
+	 * Number of sectors used by catalog.
+	 * FIXME: some sources say 10, others say 11. RDOS 3.3 may support 16.
+	 */
+	public static final int CATALOG_SECTORS = 10;
 	/**
 	 * The known filetypes for a RDOS disk.
 	 */
 	public static final String[] filetypes = { "B", "A", "T" }; //$NON-NLS-1$ //$NON-NLS-2$ //$NON-NLS-3$
 
+	/**
+	 * 13 sectors for RDOS 2.1/3.2, native sectoring (16) for RDOS 3.3
+	 */
+	private int SectorsPerTrack() {
+		if (getImageOrder() instanceof ProdosOrder) {
+			return getImageOrder().getSectorsPerTrack();
+		} else {
+			return 13;
+		}
+	}
+
+	/**
+	 * 455 blocks for RDOS 2.1/3.2, 560 for RDOS 3.3
+	 */
+	private int BlocksOnDisk() {
+		return TRACKS_ON_DISK * SectorsPerTrack();
+	}
+
 	/**
 	 * Use this inner interface for managing the disk usage data.
 	 * This off-loads format-specific implementation to the implementing class.
@@ -94,13 +117,13 @@ public class RdosFormatDisk extends FormattedDisk {
 		private int location = -1;
 		private BitSet bitmap = null;
 		public boolean hasNext() {
-			return location == -1 || location < BLOCKS_ON_DISK - 1;
+			return location == -1 || location < BlocksOnDisk() - 1;
 		}
 		public void next() {
 			if (bitmap == null) {
-				bitmap = new BitSet(BLOCKS_ON_DISK);
+				bitmap = new BitSet(BlocksOnDisk());
 				// mark all blocks as unused
-				for (int b=0; b<BLOCKS_ON_DISK; b++) {
+				for (int b=0; b<BlocksOnDisk(); b++) {
 					bitmap.set(b);
 				}
 				// for each file, mark the blocks used
@@ -151,8 +174,12 @@ public class RdosFormatDisk extends FormattedDisk {
 	 * itself is a 13 sector format.
 	 */
 	public byte[] readRdosBlock(int block) {
-		int track = block / 13;
-		int sector = sectorSkew[block % 13];
+		int s = SectorsPerTrack();
+		int track = block / s;
+		int sector = block % s;
+		if (s == 13) {
+			sector = sectorSkew[sector];
+		}
 		return readSector(track, sector);
 	}
 	
@@ -166,39 +193,28 @@ public class RdosFormatDisk extends FormattedDisk {
 	 * itself is a 13 sector format.
 	 */
 	public void writeRdosBlock(int block, byte[] data) {
-		int track = block / 13;
-		int sector = sectorSkew[block % 13];
+		int s = SectorsPerTrack();
+		int track = block / s;
+		int sector = block % s;
+		if (s == 13) {
+			sector = sectorSkew[sector];
+		}
 		writeSector(track, sector, data);
 	}
 
-	/**
-	 * Read the block from the disk image.
-	 */
-	public byte[] readBlock(int block) {
-		int track = block / 16;
-		int sector = block % 16;
-		return getImageOrder().readSector(track, sector);
-	}
-	
-	/**
-	 * Write the block to the disk image.
-	 */
-	public void writeBlock(int block, byte[] data) {
-		int track = block / 16;
-		int sector = block % 16;
-		getImageOrder().writeSector(track, sector, data);
-	}
-
 	/**
 	 * RDOS really does not have a disk name.  Fake one.
 	 */
 	public String getDiskName() {
-		if (getImageOrder() instanceof ProdosOrder) {
+		if (SectorsPerTrack() == 13) {
+			/* Use the comment/tag added in the 13->16 sector conversion */
+			byte[] block = readRdosBlock(4);
+			return AppleUtil.getString(block, 0xe0, 0x20);
+		} else {
+			/* Use the name of the OS (catalog entry zero) */
 			byte[] block = readSector(1, 0x0);
 			return AppleUtil.getString(block, 0x0, 0x18);
 		}
-		byte[] block = readRdosBlock(4);
-		return AppleUtil.getString(block, 0xe0, 0x20);
 	}
 
 	/**
@@ -206,8 +222,8 @@ public class RdosFormatDisk extends FormattedDisk {
 	 */
 	public List<FileEntry> getFiles() {
 		List<FileEntry> files = new ArrayList<>();
-		for (int b=13; b<23; b++) {
-			byte[] data = readRdosBlock(b);
+		for (int b=0; b<CATALOG_SECTORS; b++) {
+			byte[] data = readRdosBlock(b + SectorsPerTrack());
 			for (int i=0; i<data.length; i+= ENTRY_LENGTH) {
 				byte[] entry = new byte[ENTRY_LENGTH];
 				System.arraycopy(data, i, entry, 0, entry.length);
@@ -250,17 +266,18 @@ public class RdosFormatDisk extends FormattedDisk {
 	 * Identify the operating system format of this disk.
 	 */
 	public String getFormat() {
-		if (getImageOrder() instanceof ProdosOrder) {
+		if (SectorsPerTrack() == 13) {
+			return textBundle.get("RdosFormatDisk.Rdos21"); //$NON-NLS-1$
+		} else {
 			return textBundle.get("RdosFormatDisk.Rdos33"); //$NON-NLS-1$
 		}
-		return textBundle.get("RdosFormatDisk.Rdos21"); //$NON-NLS-1$
 	}
 	
 	/**
 	 * Return the number of free blocks.
 	 */
 	public int getFreeBlocks() {
-		return BLOCKS_ON_DISK - getUsedBlocks();
+		return BlocksOnDisk() - getUsedBlocks();
 	}
 	
 	/**
@@ -301,7 +318,7 @@ public class RdosFormatDisk extends FormattedDisk {
 	 * Get the length of the bitmap.
 	 */
 	public int getBitmapLength() {
-		return BLOCKS_ON_DISK;
+		return BlocksOnDisk();
 	}
 	
 	/**
@@ -323,7 +340,7 @@ public class RdosFormatDisk extends FormattedDisk {
 	 */
 	public List<DiskInformation> getDiskInformation() {
 		List<DiskInformation> list = super.getDiskInformation();
-		list.add(new DiskInformation(textBundle.get("TotalBlocks"), BLOCKS_ON_DISK)); //$NON-NLS-1$
+		list.add(new DiskInformation(textBundle.get("TotalBlocks"), BlocksOnDisk())); //$NON-NLS-1$
 		list.add(new DiskInformation(textBundle.get("FreeBlocks"), getFreeBlocks())); //$NON-NLS-1$
 		list.add(new DiskInformation(textBundle.get("UsedBlocks"), getUsedBlocks())); //$NON-NLS-1$
 		return list;
@@ -418,12 +435,7 @@ public class RdosFormatDisk extends FormattedDisk {
 		byte[] fileData = new byte[rdosEntry.getSizeInBlocks() * SECTOR_SIZE];
 		int offset = 0;
 		for (int blockOffset = 0; blockOffset < rdosEntry.getSizeInBlocks(); blockOffset++) {
-			byte[] blockData;
-			if (getImageOrder() instanceof ProdosOrder) {
-				blockData = readBlock(startingBlock + blockOffset);
-			} else {
-				blockData = readRdosBlock(startingBlock + blockOffset);
-			}
+			byte[] blockData = readRdosBlock(startingBlock + blockOffset);
 			System.arraycopy(blockData, 0, fileData, offset, blockData.length);
 			offset+= blockData.length;
 		}