Let's briefly recap the evolution of the HDD Synth prototypes 

MKI

My first PCB designed in KiCad and manufactured by PCBWay

MKII

Quickly followed by a refined version also using breakout boards for the amplifier and SD card reader which required hand assembly

MKIII

Switched to use surface mounted components and eliminated breakdown boards. Gearing up for mass production.

MKIV

A smaller version with an integrated CompactFlash reader, designed as an all in one solution

MKV

Well...

The MKV uses the same 2.5" HDD form factor as the MKIV, however it's designed to be used in any type of computer and with lower manufacturing costs than previous models

USB-C

The first few versions were ISA based which is where they got their power from, however this limited their use to only ISA based computers. If you only have PCI or Zorro slots then you'd be out of luck

I also had a few people expressing an interest in being able to use it in modern computers.

As I researched different types of power connectors I ended up deciding to use USB C as a universal type. This allows direct use with USB C or via an adapter, such as https://thepihut.com/products/usb-c-to-4-pin-molex-socket-cable-0-5m

Essentially it just needs a 5V supply, so by using a USB C to X adapter it can be connected up to any computer with a 5V rail

Onboard RP2350

The MKIII took a leap forwards by integrating all the original breakout boards for the amplifier and SD card reader onto the PCB itself. The next logical (and more complex) step was to do this for the Pi Pico as well. So to use the RP2350 chip directly, which should cut costs and board size.

JLCPCB

For the MKV prototype I'm using JLCPCB for the first time to manufacture it. PCBWay has been a really accessible and supportive partner in getting started with PCB production, however I'm keen to drive down costs as far as possible which I believe JLCPCB will help me do.

Kickstarter

If everything works as designed (fingers crossed!) then after this the final step will be having a Kickstarter in order to sell the retail version of this. It'll be great to get the device into the hands of people finally to enjoy