Are we talking about Virtual Disks or Physical Disks. I wouldn't recommend taking a disc image of a hard drive with Virtual Discs. A simple copy/paste of a virtual disk would suffice. They are very easy to move between computers.
The thing about OS Agnostic ones is that they require the machine to be shutdown. They basically take a sector-by-sector image of the hard drive. The ones that Acronis and Macrium sell runs while the computer is on using their own VSS writers or the built in one on Windows. So it's not really an alternative to backing up a server. Almost all of them charge more to backup a server. I use the Windows Server Backup tool. I've been able to successfully the restore the image to a bare metal on a few occasions.
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