FW 800 external drive mounts on OLD Mac Pro but not on NEWER iMac

Apple has a Thunderbolt to Firewire adapter that uses the old-style Thunderbolt connectors, and a Thunderbolt 3-to-2 adapter that can plug into a Thunderbolt 3 or 4 port. You could daisy-chain the two.

However,

• An iMac (Retina 5K, 27-inch, 2020) does not have a Lightning port. It has two USB-C (Thunderbolt 3) ports and four USB-A (USB 3) ports.
• I've never heard of a Lightning to Firewire adapter – and don't think you could build one that actually worked.
• Any "USB-A to Firewire" adapters that you come across should be avoided like the plague. If you are lucky, the "adapters" won't work. If you are unlucky, they may cause electrical damage to your computer and/or drives. It isn't a good idea to randomly connect wires between the pins of completely different types of interfaces.

If you are using daisy-chained Apple adapters (TB3-to-2, TB-to-FW800), things should work. I don't know why they wouldn't.

If you are using any sort of purported "Lightning to Firewire" or "USB to Firewire" adapter, I would question how you managed to plug it in, and whether the bogus adapter might actually be endangering your equipment.

You can try an Apple USB-C Thunderbolt 3 to Thunderbolt 2 Adapter combined with an Apple Thunderbolt to Firewire Adapter, plus an appropriate Firewire Cable as well. No idea if the Apple branded adapters will do any better than a third party option, but I have used this setup for a couple of old Firewire devices. I've had mixed results using third party adapters with the USB-C Macs.

Seems like the most economical option would be to use an older Mac to transfer the data to a newer USB 3 or Thunderbolt 3 drive or to an intermediate location so that you can transfer the files to the newer drive used on a newer Mac.

P.S. – Since those drives have both Firewire 800 and USB 3.0 interfaces, you might want to consider using USB 3.0 with your new iMac. There are various ways to attach USB-A devices to your USB-A and USB-C ports. You would want to use a cable rated for USB 3.0 or faster speeds.

The main drawback I can see is that you might get S.M.A.R.T. drive health status through a Firewire connection that you would not get through most USB ones. Switching to USB might make that status unavailable.

But at this point, I would think you would want to use Firewire mainly for old MiniDV / Digital8 camcorders or for other old audio/video gear where moving over to USB 3.0 (or better) isn't a realistic option.

The other thing that comes to mind – does that Quad drive rely on any special driver that might be present on the old Mac, but that is not present on (or not compatible with) the new one?

E.g., any sort of LaCie software to make all of the four drives work together as a RAID? Although that would seem to be contraindicated if the drive works with a USB connection …

USB 3 is actually faster than FireWire 800 and I would use that instead.