How do I connect firewire 800 hard drive to my new imac

That's a very difficult question to answer. Apple makes a Thunderbolt-to-Firewire 800 adapter but makes it clear that this adapter works with Thunderbolt and Thunderbolt 2 ports. Your new Mac portably has Thunderbolt 3 ports. Apple also makes a Thunderbolt 3-to-Thunderbolt/Thunderbolt 2 adapter. Using those two adapters in series would get you to Thunderbolt 3-to-Firewire 800 BUT I don't know if it would actually work or not. There are a number of Thunderbolt 3 Docks on the market that include a Firewire 800 port but they are not cheap. It all depends upon how important it is for you to keep those Firewire 800 drives in service. It would likely be more cost effective to buy a new USB 3.1 or Thunderbolt 3 hard drive to replace the Firewire 800 old one.

Which drive?

All of the newish FW800 drives I've seen or owned were dual FW800/USB 3.0, so when migrating I simply used my drive's micro-USB-B to USB-A cable instead of firewire. Is that an option for you?

What brand and model is your current FireWire external drive? Some enclosures are easily opened and the drive inside can be placed in a more modern enclosure with USB 2 or USB-C. That way the data is preserved.

I agree with Alan's approach, if you can extract the drive from the enclosure OWC (www.macsale.com) makes a number of enclosures that would work. I intend to buy an OWC Thunderbay 4 with Thunderbolt 3 (USB C) to connect to my next iMac. You can get the Thunderbay 4 in a JBOD (Just a bunch of disks) set up or if you want to make a RAID setup you can do that too. I'm going to move all the data on my Firewire 800 drives to a RAID setup on my Thunderbay. It's not cheap but it's a great approach allows you access to all of your old data and is FAST!!!!

Check www.macsales.com , they have Thunderbolt 3 (USB C) enclosures available that come with no HD inside. For example the Thunderbay 4 & 6. You can get them with HD's installed or without.

Yeah’ I have two Mini Stacks that we’re connected to my old Mac via FireWire 800 and I replaced their cables with USB 3.0 cables. Just look on the back of the Mini Stack. If the square USB connection is blue, then you need a matching USB 3.0 cable. If the square USB connection is white, then just get a matching USB 2.0 cable.

I use those same adapters and it works fine for scanners I have that are FireWire 400 and 800.

I’d be happy to try my cable and Apple FireWire adapters, but I retired all my FireWire drives years ago, sorry.

Kinda funny this post.

I guess I am not the only one still using an aging iMac that still uses FW800 in 2020.

I have/use three external FW800 drives with my “still going”, late 2009, 27 inch screen iMac and planing shortly on getting a fourth with another SSD inside.

I'll be running into this same issue if I ever purchase a much newer Mac.

I guess I am not the only one still using an aging iMac that still uses FW800 in 2020.

The TM backup on our geriatric 2011 iMac is still via FW800. Faser than USB2! Until recently the external was a OWC Neptune that was FW400 only.