Internal SATA connection or external (and what type)?
I'm using a StarTech multi-drive docking station connected by USB 3.0, so that may be one cause.
Also, I know the dock's firmware needs upgrading (just haven't had the time to do so), since I was having really slow speeds (> 12 hours) formatting a 1TB HGST Travelstar. When I moved that drive into an internal laptop bay it formatted in about 2-3 hours.
The 4TB drive that I first formatted was also an HGST, so maybe the old firmware has problems correctly recognizing the specific configuration of the HGST brand of drives. When it first took that 30 hours, I wasn't that surprised, since all of the drives I had put in that dock before were WD 500GB (and I didn't remember exactly how long it took to format those, since I would typically format them while I was away from the machine, and also it's been a long time since I formatted any of those 500GB drives). But 30 hours for a drive 8 times the size of my previous drives didn't seem so extraordinary.
This now gives me more incentive to upgrade the firmware and try formatting the next 4TB drive. I'll post the result here.
Lightning, thanks for the info!