This is also not related (directly) to firewire. It's the old interface card - the support chip inside the d2 enclosure.
If the interface chip does not support newer, larger drives (more than 2TB), one of the symptoms is that you get that 800GB. In this case (pun intended!), Disk Utility will show the drive as 800GB [3TB (real capacity) - 2.2TB = .8TB = 800GB]
And the reason is that, assuming that firewire is used, the firewire support chip on that d2 interface doesn't support those big drives.
The current d2 Quadra (which still includes Firewire 800) comes with either 4 or 6TB. So, not a firewire limitation, per se, but a limitation of the interface card used in the older model d2 that the OP has. (the current Quadra also has USB 3, which is significantly faster than the FW800, but NOT on a 2009 iMac - still USB 2 on that, so FW800 is, at least for speed, a better choice on that old Mac.
I am almost always connected to a Firewire 800 storage drive, and don't experience any sleep problems with that. But, it's just a storage device. Maybe your issue is the connection to other kinds of devices (like audio) that I don't ever use, at least on firewire.