@butch
https://www.tascam.eu/sw/if-fwdmmkii/IF-FWDM_MKII_188754_Installer.dmg
This is the driver that worked for me recently on High Sierra. If you aren't already aware, with OS X High Sierra there are a few security hoops you have to jump through each time you install software that isn't "known to Apple." i.e. isn't something from the App store or coming from a major software developer like Waves, AVID, ect.
During the install process, if you open the "System Preferences" panel and go to the "Security & Privacy" section, you should see the "General" tab and at the bottom it will give two options, with buttons to select either:
"Allow apps downloaded from:
1. App Store
2. App Store and Identified Developers
Choose the second option. (You may need to input your Admin password to unlock the settings, by clicking the lock icon in the bottom left.)
After that, anytime you install software, drivers, extensions, ect. that are not known by Apple (Such as the drivers from Tascam) you will be asked to approve their installation at this window.
Once you attempt to run the installer, there should be a button that isn't always present that will pop up on this page that says "Open Anyway". Click that to allow the process to proceed. Sometimes you will have to restart the installer program, but not always.
And you will almost certainly require a restart of the system for the install to take effect, even if not prompted to do so by the installer program.
This is a security measure that has plagued many users since High Sierra was launched, not only with Tascam software, but virtually anything that was not a "known developer" according to Apple. So if you plan to stay on that OS, just know that it can be causing this problem each time you try to install software or plugins that are from small or relatively niche devs.
Hope this helps. My entire setup is based around High Sierra, so I've had plenty experience jumping through its various hoops on my way to creating a stable system, which it very much is.
Best of luck,
Zach
P.S. Even though this is the Firewire section of the forums and you are technically looking for help with a FW device, I would recommend searching threads on this topic in the "Dm3200/4800" section, if you haven't already. Cheers!