Fix for handle 256bytes sectors format not to use fast seek

src/BlueSCSI_disk.cpp

@@ -2237,17 +2237,32 @@ static void start_dataInTransfer(uint8_t *buffer, uint32_t count)
     // Use direct sector I/O when fastseek is enabled (for fragmented files)
     // Contiguous files already use raw SD access, bypassing this path
     bool read_ok = false;
-    if (img.file.isFastSeekEnabled())
+    uint32_t sectorCount = count >> 9;
+    uint64_t filePos = img.file.position();
+    bool posAligned = ((uint32_t)filePos % SD_SECTOR_SIZE) == 0;
+    if (img.file.isFastSeekEnabled() && sectorCount > 0 && posAligned)
     {
         // Convert byte position/count to sector units (>> 9 is / 512)
-        uint32_t fileSector = img.file.position() >> 9;
-        uint32_t sectorCount = count >> 9;
+        uint32_t fileSector = (uint32_t)filePos >> 9;
         uint32_t sectorsRead = img.file.readSectorsDirect(fileSector, buffer, sectorCount);
         read_ok = (sectorsRead == sectorCount);
         if (read_ok)
         {
-            // readSectorsDirect does NOT advance file position - do it manually
-            img.file.seek(img.file.position() + count);
+            uint32_t fullBytes = sectorCount * 512;
+            uint32_t remaining = count - fullBytes;
+            if (remaining > 0)
+            {
+                // Partial SD sector at end (e.g. odd number of 256-byte SCSI sectors)
+                // readSectorsDirect does NOT advance position, so seek past full sectors
+                // then use filesystem read for the remainder
+                img.file.seek(filePos + fullBytes);
+                read_ok = ((uint32_t)img.file.read(buffer + fullBytes, remaining) == remaining);
+            }
+            else
+            {
+                // readSectorsDirect does NOT advance file position - do it manually
+                img.file.seek(filePos + count);
+            }
         }
     }
     else

src/ImageBackingStore.cpp

@@ -303,6 +303,13 @@ ssize_t ImageBackingStore::read(void* buf, size_t count)
     uint32_t sectorcount = count / SD_SECTOR_SIZE;
     if (m_iscontiguous && (uint64_t)sectorcount * SD_SECTOR_SIZE != count)
     {
+        // Keep FsFile and raw-sector cursor in sync before changing access mode.
+        uint64_t pos = (uint64_t)(m_cursector - m_bgnsector) * SD_SECTOR_SIZE;
+        if (!m_israw && m_fsfile.isOpen() && !m_fsfile.seek(pos))
+        {
+            logmsg("---- Failed to sync FsFile position during contiguous read fallback");
+            return -1;
+        }
         dbgmsg("---- Unaligned access to image, falling back to SdFat access mode");
         m_iscontiguous = false;
     }