There was a Super Expander available with harddisk, SVI-609 (SVI-605 with 10MB harddisk), but I don't have any information about it.
Another way is to build your own harddisk interface, the simpliest to build would be an IDE (also known as ATA) interface. There is a lot of resources on the net for building a simple IDE interface for the Z80.
It is also possible to build a communication device like RS-232 or EPP and then use a PC as a "server".