Best type of disk image to use on NAS to archive Mac system copies on a NAS

Solution 1:

For archival images, I'd use the "compressed" format (which I think you'd consider "fixed", but it's one of a number of fixed-size formats). The sparse formats are useful for when you want the image to be able to expand later, when you add to them... which is pretty irrelevant for archived systems. There's also a fixed-size read-write format, and a read-only (uncompressed) format, which'll store the same thing, but take up more space for no good reason.

All the versions you mention will be in HFS+ (aka Mac OS Extended) format. In my experience, the best way to image an HFS+ volume is to mount it (note: don't try to image the volume you're running from at the time), open Disk Utility, choose File menu > New Image > Image from Folder, and select the top level of the volume as the "folder" to image. In the save dialog, make sure Image Format is set to Compressed, and it's best to save it somewhere local (and copy it to the NAS afterward). Once it's finished, choose Images > Scan Image for Restore, and have that process the newly-created image. Then copy it to the NAS.

Why image "from Folder" rather than from the device? Because it creates a cleaner image (files all defragmented, etc) but preserves everything that matters (i.e. if the original was bootable, you can restore the image and the result will be bootable). It also makes the image just big enough for the files, since it doesn't include all the empty space in the original volume. Including the empty space can cause trouble if you ever need to restore it to a smaller volume.

Why "Scan Image for Restore"? So you can use Disk Utility's Restore feature to restore it to a disk later (or use the command-line asr tool to do the same thing). You could do the scan process later, but I prefer to do it immediately, so the image'll be ready to go if I ever need it.

Solution 2:

The simplest option is often also the most reliable - I would use a simple bit-for-bit identical copy (perhaps this is what you call a "fixed disk image").

Such an image can be written back to the physical hard drive again and be able to boot up. You'll also be able to mount the file system inside the image as a virtual, read-only drive in order to inspect and copy files.