Firewire to usb 3.0

Mac 7,1
ports: USB 2.0 and Firewire
I need a 3tb external hard drive for time machine back up. Firewire hard drives are no longer available. USB 2.0 is unacceptable due to 20+hour back up. So, how can I get from a firewire port to a 3.0 hard drive? Or does someone have a different suggestion?
thank you
i don't think my first post was posted, so if this is a duplicate then i apologize

I've seen numerous devices, mostly cable adapters, that have firewire connections on one end and usb on the other. I'm skeptical that any of these would work in you case because of some of the hurdles to be overcome. See here for example.

How did you arrive at the 20+ hour estimate of backup time. Just how much data is being backed up?

BTW assuming that the 20+ hour estimate is correct subsequent backups won't take nearly that long or the Time Machine backups couldn't occur hourly.

OWC do up to 2TB in USB2, USB3 and FW800:-


OWC Mercury On-The-Go Pro USB 3.0 & 2.0 / FireWire 800, and 2.5" SATA Portable External Hard Drive & SSD


Western Digital do a 3TB external FW800:-


http://www.amazon.com/dp/B004QGXOP2/?tag=macforums0e4-20

Firewire and USB are not compatible. There is no adapter that will transfer data between the 2 of them.

The WD Harry pointed to above is one option - that drive was discontinued right about the same time Apple quit providing FW ports on Macs. I bought one not too long after WD made the announcement and still using it as my iTunes media drive. The retailers put them on sale back then, now you'll be paying a premium for one of the few still left.

OWC will be the best bet. They're still putting out FW enclosures for those that haven't upgraded their Macs yet. Hit the external drive link on the page harry posted, they have several FW enclosures, with a drive or bring your own.