Compact Flash card interface - 8-bit, full capacity, very simple
Schematic  Programming reference - LBA mode   Download new ROM - LBA, big sectors, for drives over 500MB.   Download ROM - LBA for smaller drives.  
ROM source (LBA)  Instructions and software for CF init.  Demonstration AVIs: Highway Encounter start, snapshot usage (3MB)    Popeye (2.5MB) - real Speccy video captures, not emu.

 Movie  (3MB) - 1 minut clip, converted with BMP2SCR. Playback is at exact 25 frames/sec, what means that data transfer rate is 175KB/sec (1 frame is 7168 bytes, multiple of 512 as sector size)

IDE hard disk interface - 16 bit data transfer
Schematic - note: here is Spectrum Edge connector layout.  Zxide.gal ... Zxide.jed 
Download new ROM - LBA, big sectors.  Download ROM - CHS, with sources(25KB)  Sources in ASM format - HD init, Floppy to HD, HDDOS (7KB) ROM usage Programming reference    Spectrum 128K compability
Pic. of interface  Pic. of Speccy with old hard disk
Simpler interface - 8bit data transfer   Download new ROM - LBA, big sectors of 1KB, for use of bigger drives up to 4GB.    Download ROM - CHS.

 IDE-Flash or Compact Flash instead hard drive
Today is probably much wiser to use some Flash drive instead classic hard disk. It consumes much less power, is very small, and prices are not big. I tested above 16-bit IDE interface with Transcend's 128MB IDE-Flash drive (128MDOM40V). Current price is about 30 Euro, what is bigger than price of Compact Flash, but you don't need IDE-CF adapter. It works very well, measured read speed is around 150KB/sec, access time is under 1ms.
Compact Flash (CF): interface worked well with SAN CF card. With Kingston I had problems - corrupt data. Solution is putting 1Kohm resistor on line SELP (pin 37 of IDE conn.) to prevent ringing. Transfer speed of 128MB Kingston CF is same as with Transcend IDE-Flash. Download ROM, instructions, sources (27 KB)

UVY-RGB converter for 48K models - get sharper & cleaner picture Schematic

ROM switch - select from diverse ROM versions Description & schematic   Zxeprs.gal

Flash EPROM instead ROM Schematic & description

Floppy interface
Schematic  ROM image with brief instructions & format program How to use ROM   File format structure

External ROM modul  - Schematic

  NMI button

For testing: All ROM versions and HDF images for them with 3 prgs.
ZX: Read hard drive parameters (CHS, size)  Copy from one hard disk to other   Disk initialization  Binary data packer (compressor)  Tape to hard disk copier  Hard disk usage statistic  StartPack - 6 MB rar archive - image for 128MB disks - many games, some utils.
PC: (USB) Drive image & ZX file transfer util.  Instructions, screenshots - Linux version^NEW
  Floppy imaging prg with ZX floppy data transfer.
ST: From floppy or disk image to TAP, Z80.