First, you'll need to buy older 3.5" 720K DD external disc drive with "READY" signal, Side Switch fitted, 34 way IDC Male connector on 34 way ribbon cable and power supply. It's very hard nowadays to find one, but you can try to get it from John King, Arthur Cook & Ebay, if available. Usually, it comes with 34 pin PCB connector onto 34 way ribbon cable for connecting to Amstrad CPC 664 / 6128 instead.

• If got 3.5" disc drive with 34 pin PCB edge connector on 34 way ribbon cable, follow the READY signal instructions if you own CPC 6128 (like me) or if you don't have CPC 6128, skip to the next step. Otherwise, if you're very lucky that you got 3.5" disc drive with 34 IDC Male connector on 34 way ribbon cable, you can go straight to step 4.
  

• Next thing is to leave at 20cms away from the PCB edge connector and then clamp the 34 way IDC Male connector onto it – the Connection Pin Numbers are all the same as CPC 664 / 6128. Making sure that the arrow on the 34 way IDC connector is clamp on from the red strip. Now you have a option to either cut the 34 pin PCB edge connector off completely (as I prefer) or leave it with 34 way IDC Male connector, making it multi-choice 3.5" disc drive to use on CPC 464 or 664 / 6128.
  

• Connect the 3.5” disc drive onto drive B connector on disc interface (back of DDI-1), switch on DDI-1 first, plus switch on 3.5" disc drive as well, then the computer. follow the READY signal instructions again but this time go to step 2. If you end in step 3A, then you have successfully add 3.5” disc drive B to DDI-1. Congratulations!!!
  

• Now try to format / copy files onto 3.5 disc.
  

NOTES

• I was the first person to find out how to add 3.5" disc drive to the back of DDI-1, tested it to see it worked perfectly and write out the instructions how to do it, as I always wanted to add 3.5" disc drive with DDI-1 to my CPC 464 Computer from Wed 4/7/2001
  

• 34 way IDC Male connectors - 34 pins sticking out, which it looks very similar to the connector on the back of DDI-1 or FD-1 disc drive. Its very rare and more harder to find one of these connectors nowadays, but is useful if you have broken DD-1 3" disc drive, basically, just remove the connector from the back of DD-1 3" drive and add it to the 3.5" disc drive instead. Try to get one from Arthur Cook, if you're lucky, that if he got more of these connectors, otherwise you'll be very disappointed.
  

• To clamp these IDC or edge connectors onto the ribbon cable, you'll need to buy Velleman 6mm to 6mm IDC Crimping Tool, which it can be bought from ebay. Please note, these are not available in the UK, believe me, I have tried, looking the whole London for this IDC Crimping Tool, including Maplin, but no luck! Unless you you have to order it from America, and its the only country that you'll ever see these IDC Crimping Tools around.
  

• Also, older 3.5" 720k DD disc drives are very hard to find as well, best to grab one from BBC Master ("CUMANA DRIVE 0" - NEC FD1036 A) computers, as these has a "READY" signal, and then convert the drive cable to work on the Amstrad CPC, as its allowing you to able to transfer data from PC to CPC and visa versa, but as for Amiga & Atari ST drives, I don't think that it has a "READY" signal.
  

• Pinboard Computer SWITCHABLE B drives (TEAC FD-235HF) has a "READY" signal, but it doesn't allow you to transfer any data from PC to CPC and visa versa, as it cannot read MSDOS formatted discs, when it suppose to.
  

Here is a list that I have experienced with the following disc drives:

3.5" READY Signal Test

(my very favourite testing stage for 3.5” disc drives)

1. First of all, make sure that 3.5" external disc drive B is connected to the back of "Disc Drive" connector (located on the back of keyboard), and switched on. Then power up the computer by switching the monitor on and the keyboard on as well, the 3" and 3.5" drive lights up, ready for 2 disc drive operation.

2. Put CP/M Plus disc (Side 1 of CPC system discs) on Drive A and no disc in drive B (3.5” disc drive). Type |CPM, then press ENTER.

3. Now it starts booting CP/M Plus. Make sure that you see this message “2 disc drives” on the screen during CP/M Plus start-up

A. If you see this message (as shown above) on the screen with CP/M Plus disc on Drive A (3”) and no disc on drive B (3.5”), this means that your 3.5” external disc drive has a READY signal. EXCELLENT!!!

B. If you see this message instead "1 disc drive" on the screen, then it means that your 3.5" disc drive doesn't have a READY signal. That is very inconvenient because you have to put a disc in Drive B as well every time when you boot CP/M Plus to use both disc drives (e.g. using DISCKIT or PIP to copy discs), but its good for CPC 664 (a useless computer), as it can't run CP/M Plus, it'll run CP/M 2.2 instead.

4. Switch everything off. Now you know the easy way how to check the 3.5" disc drive to see it has READY signal or NOT!!!

Amstrad CPC 464 (6)