[rjbell]

I would like a HDD player for my hi-fi system but they all cost over £500. Could a dap be modded to run off a HDD rather than a SD card?

 

[markbrauer]

How about using a notebook/netbook like this
https://www.amazon.co.uk/Lenovo-ideapad-11-6-Inch-Laptop-Notebook-x/dp/B015Q27Z1S/ref=sr_1_2?ie=UTF8&qid=1483642090&sr=8-2&keywords=windows%2Bnetbook&th=1
and a highly-rated USB DAC like this
https://www.amazon.co.uk/AudioQuest-Dragonfly-Rosso-Audioquest-Black/dp/B01DP5JHHI/ref=sr_1_1?ie=UTF8&qid=1483714030&sr=8-1&keywords=dragonfly+dac
Considerably less than £500. 
Add some music player software and connect your USB HDD. You should get very high quality sound. The computer is fanless so it adds no noise to your listening environment. Most player software today allows remote control via a smartphone so once set up you could run it "headless". The WiFi would give you the option of streaming from the net, which DAPs generally don't do.
 
I use an old laptop that a friend gave me in a similar fashion. It sits behind the other components and just does it's thing. All control is from my Android phone.
 
A very inexpensive option would be a player built up from a Raspberry Pi or other headless computer. Of course that takes a desire/ability to dig in and do some tweaking. Here's one option.
http://www.runeaudio.com/
This would be my choice if/when my used laptop quits.