I'm trying to create a Linux installation drive. I have an empty hard drive (MS-DOS format), and I already dragged the .iso file to the sidebar in the Disk Utility app. However when I press it the "Burn" option is still greyed out. Also, when I double click the .iso file it says "no mountable file systems". Does anyone know the solution to this?
I don't think Disk Utility will let you "burn" an ISO to a hard drive. It might let you "restore" the content to the drive. Select the ISO image in the right-hand side, then drag and drop the hard drive into the "Destination" field and click "Restore". I don't have a device here to test, but that might work.
Normally people "burn" ISO images to CDs or DVDs, or write them to USB memory sticks. If you have one of them available here are the relevant instructions for Ubuntu (these instructions should work for other Linux distributions too). I've done both countless times and can confirm that they do work.