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iBored is basically a hex editor for disk sectors, but can also be used to edit files, including disk images.

This program comes with hardly any documentation, and it doesn't explain what it does, either. If you have not used other tools to edit disk blocks or files on a binary level before, you may find it difficult to use.

Beware: This tool lets you mess with your disk contents in a rather unsafe manner: You can easily write over any data on a disk, rendering the entire disk unusable. So you better know what you're doing, especially if you use any "write" operations. And keep backups of your data on other disks.

Features

Can view disks in custom block sizes.
Deals gracefully with read errors: You can copy a partially-damaged disk to an image file (bad blocks will become zero-filled in the copy) in order to use recovery tools on the error-free copy then.
When modifying data, a "journal" file is written to your desktop that contains the previous data of the altered blocks, and it can be used to undo your changes.
Can view partitions and other subranges as containers with their own start, length and block size.
Can be re-launched with admin (root) rights to access non-removable disks (Mac OS X only).
Detects connected iPods, showing their firmware partition contents.
Can access disks remotely over a network connection.
Can save a range of blocks to a file and write a file back to disk.
Has a powerful template system to view and analyse disk structures.
Can edit MBR and GPT partition tables.
Can install a PC BIOS bootloader dealing with both MBR and GPT.