If your device can make the maximum theoretical speed of 800 Mbits/sec, then the copy should take about 3 1/2 minutes.
but, other things come into play here. What condition is your drive? How fast can it transfer information in and out?
A bunch of mid-sized files totalling 21GB might copy faster than one single 21GB file.
A large number of pretty small files might take longer.
Are you transferring FROM a FW800 device to your internal drive, or to another external, also on FW800? Or to another external on USB? Is it USB 3, or USB 2.0 (some USBs will automatically downshift to slower speeds.
Are you copying TO a USB flash drive? (can be really slow, depending on the flash drive, much longer than 12 minutes)
Anyway, 12 minutes is not exactly out of line, but if everything is working in the hardware, then you should see somewhat less than that.