How to Create Images with Yosemite
If you are able to mount the drive, you can use Copy Copy Cloner or even better (IMO) SuperDuper! from Shirt Pocket. SuperDuper! will not copy caches and unnecessary files that are re-created upon boot, saving space. These both give you clones of the drive, but not images. If you can restore the clone, then this might work.

If you're savvy with a Terminal, you can use the basic diskutil and dd commands to get a disk image - this is what Disk Utility is likely doing under the hood. Just be careful with the input/output parameters of the command, to ensure you don't overwrite something you're not supposed to.

For example, you can find a disk and the partition you want with:

diskutil list


Then, to unmount the disk:

diskutil unmountDisk /dev/disk&lt;n&gt;


where the "&lt;n&gt;" is the disk number of the drive you want to image. Finally, create an image using some form of:

sudo dd if=/dev/disk&lt;n&gt;s&lt;p&gt; of=/path/to/image.img bs=1m


where the "&lt;n&gt;" is the disk number and "&lt;p&gt;" is the partition number you found above. Do not type any of the brackets (&lt;&gt;) as part of the command. There are other options you can add, and you can adjust the block size (bs) accordingly.

I just found more information at another post here: What is the best way to clone a disk between two Macs?