ATA Interface for PIC
I wouldn't recommend it outside of an academic exorcise.  You'll need 24pins (16 data 8 interface) and most likely bit bang the interface protocol as i don't think there is a PIC18 with any sort of 16bit parallel port.  Bit banging the interface, managing the ATA command layer and managing the file system could consume almost all the PIC's time if your trying to get any sort of reasonable transfer rate.

If you really want to go this route a quick google search for "PIC IDE" came up with a few places to look for code that supports ATA/IDE on PIC's.

www.pjrc.com/tech/8051/ide/wesley.html 

www.telegraphics.com.au/sw/info/picide.html

Alternately, interfacing to a SD card is really easy via a SPI port and there are PIC18's with SPI interfaces with DMA that would require much less work from the PIC to manage the interface.

If your worried about reliability there are industrial grade SD cards and board mount locking SD connectors.