I have a usb2/esata external drive I am using for backup via usb2
on several machines.

The esata works also, but I can't figure out if there is a way to
rescan the bus and attach the drive outside of a reboot.

Can drvctl(8) and atactl(8) be used to attach and detach the esata
drive in this manner? How do you configure drvctl to do this?