According to the source code at the following URL, Magic Number is the identifier of the source image * constant. This value is then used to check the long int. If the int was not as expected, it was considered not synchronized with the image environment.
http://grepcode.com/file/repo1.maven.org/maven2/org.robolectric/android-all/4.4_r1-robolectric-0/android/media/MiniThumbFile.java#MiniThumbFile.getMagic%28long%29
// Get the magic number for the specified id in the mini-thumb file. // Returns 0 if the magic is not available. public synchronized long getMagic(long id) { // check the mini thumb file for the right data. Right is // defined as having the right magic number at the offset // reserved for this "id". RandomAccessFile r = miniThumbDataFile(); if (r != null) { long pos = id * BYTES_PER_MINTHUMB; FileLock lock = null; try { mBuffer.clear(); mBuffer.limit(1 + 8); lock = mChannel.lock(pos, 1 + 8, true); // check that we can read the following 9 bytes // (1 for the "status" and 8 for the long) if (mChannel.read(mBuffer, pos) == 9) { mBuffer.position(0); if (mBuffer.get() == 1) { return mBuffer.getLong(); } } } catch (IOException ex) { Log.v(TAG, "Got exception checking file magic: ", ex); } catch (RuntimeException ex) { // Other NIO related exception like disk full, read only channel..etc Log.e(TAG, "Got exception when reading magic, id = " + id + ", disk full or mount read-only? " + ex.getClass()); } finally { try { if (lock != null) lock.release(); } catch (IOException ex) { // ignore it. } } } return 0; }
I got an exception at runtime while trying to get the original thumbnail id by looking at the thumbnail track. (BTW, the disk is not full and it is not read-only.)