Save As Disk Image
This option is available for disk images and partitions (not filesystems). It allows you to save the contents to a new disk image. The new disk does not have to use the same format, so this is a quick way to convert a disk image from one format to another.
Select the disk image or partition in the archive tree, then either click the Save As Disk Image button in the center panel, or use the corresponding item in the Actions menu. This opens a window that lets you pick a file format. Options that aren't appropriate for the disk's size will be disabled. Pick an option and click Save, then use the file dialog to pick a name and location for the new file.
Converting a block/sector image to a nibble image will generate standard low-level formatting on the new disk. It should behave identically to the original.
Converting a nibble image to a block/sector image will discard the low-level formatting, and might partially fail if the source disk has errors or copy protection. Any read errors encountered will be reported during the transfer, but the loss of certain data (like the volume numbers embedded in the sector address headers of 5.25" disks) will not be called out.
Converting a nibble image to a different nibble format will still result in the loss of data, because the transfer is always done at the block/sector level.
Replace Partition Contents
This option is only available for partitions. It allows you to overwrite the contents of the partition with those from a disk image. The filesystem on the disk image must be the same size or smaller than the partition.
Select the partition in the archive tree, then either click the Replace Partition Contents button in the center panel, or use the corresponding item in the Actions menu. Use the file dialog to choose the disk image file. A window will open showing the geometry of the disk image (source) and partition (destination), and asking for confirmation. If the geometry of the two is not compatible, you will not be allowed to continue. If all is well, click Copy to overwrite the partition. Otherwise, click Cancel.
Important: this overwrites the entire partition, completely replacing the previous contents with the disk image. It does not merge them together. Anything in the partition will be destroyed.
Scan For Bad Blocks
This option is only available for disk images that use a nibble format (e.g. .nib and .woz). It scans the disk for bad blocks or sectors, and reports a list of failures. This is a non-destructive scan; it makes no attempt to fix problems found.
This does not look for filesystem issues. Those are scanned for automatically whenever the disk image is opened.
Defragment Filesystem
This option is only available for filesystems that have serious problems with fragmentation (this is technically a filesystem utility, not a disk utility). This is currently implemented only for Apple Pascal.
Select the filesystem, then select Actions > Defragment Filesystem from the menu. The operation runs immediately.
For safety reasons, you will not be allowed to defragment a filesystem that has any errors or irregularities.