I'm actually unaware of AHCP. Did you accidentally mean AHCI (Advanced Host Controller Interface)?
Just to clarify, do you have an external enclosure with USB AND eSATA?
eSATA from my experience is plug and play and hot swappable when the HD is turned on, as I've booted from my vantec enclosed sata hd- so you wouldn't need any drivers.
If you're really desperate or the eSATA really doesn't work, you can always run it from one of your other mobo SATA ports (with an open case) and see if that works (that's what I do at the moment).