Since a very long time, I had my old 4th Generation iPod laying around. With iPhone, SoundCloud & Co, I hadn’t had any use for it - until now: my Camping Van came ‘only’ with a CD Radio and an Aux-In - which is perfect for my Ipod. If it would only work. Battery was down, and even worse - the Harddisk crashed. But a quick google made me hope: there’s indeed a chance to replace the HD with Flash memory. Faster, cheeper and less power consumption. I had to try it.
So I followed those super easy steps Eddie posted on instructables:

http://www.instructables.com/id/Convert-your-4th-Gen-iPod-to-use-Flash-Memory

Main trick is to get a ‘IDE 50 Pin Male zu CF Compact Flash Female Adapter’ - on ebay or similar for just 5 EURs. Compact Flash memory you get for abt. 1EUR per GB, which int total, made the 40GB replacement quite cheap. It just took me minutes to replace the harddrive, including a new Battery. Now my more than 10 years(!!) old iPod works better like never before! Amazing.

Happy Hacking!

UPDATE: make sure you check DIAG-Mode and HW-Reset too:

First what I would do is wait for it to reboot itself and as soon as the screen turns off begin holding the Center + Play/Pause buttons until Your iPod Enters Disk Mode and THEN plug it into your iMac and select “Restore”. =) Also when you get the chance, Hold Menu + Center and as soon as the screen goes out, Hold Center + Previous/Rewind. You will get a White Screen, select “Manual Test” and then go to “IO” then “Hard Drive” and then select Hard Drive Smart Data and type every character on that screen up here, I will be able to tell you if your iPod’s internal hard disk is going bad

https://discussions.apple.com/thread/2609298?tstart=0