I considered Solid-State Drives (SSD) as an option but they are not easy to find or affordable, especially with the IDE interface; I also felt uncomfortable spending that kind of money on equipment this old.
As I was about to look for replacements for both laptops I came across my old camera's Compact Flash (CF) memory cards and my Compact Flash to PCMCIA adapter that allowed me to transfer the pictures to a laptop much faster. I wondered if I could put these to good use, so, to try it out, I used the smaller CF card I had (1 GB), installed Linux and made it bootable; I put it in the CF to PCMCIA adapter and inserted into the C400 hoping the laptop could boot from it. No go, no matter what I tried, the laptop and BIOS was too ancient to boot from the PCMCIA card.
I then thought " I wonder if anybody even thought of making a CF to IDE adapter"... so I Goggled it and I was pleasantly surprised that they exist and are cheap too! They ranged in price, I saw one I liked for $6.99 plus $11.00 shipping but ended ordering a couple from eBay that where $1.57 each with free shipping as I figured I was taking a chance anyway, go cheap just in case it doesn't work.
Of course this set up will not be as fast as using an actual SSD, but in my case this is not an issue as I only use the laptop to stream images from my security cameras 24 hours a day with very few reboots. The record on my C400 is 435 days running, it was stopped by a power outage since the battery is kaput.
The two items in the picture is what is mainly required, but you might need some sticky putty to fix it in place:
1 x Compact Flash to IDE adapter - (I used this one)
1 x Compact Flash card - size varies on OS requirement but you can use a 512 MB size or smaller to install a Lightweight Linux Distribution
1 x package of Instant Tac - (Got mine from our local dollar store)
Step 1: The Mating
I inserted the 8GB CF into the adapter and it is now ready to insert into the laptop.
The Dell I have has an adapter for their proprietary quick insertion connector, without this, the hard drive will not connect to the laptop.
Step 2: What the ...??? Problem!!
You can see this in the picture, the CF adapter that is in my hand has all the 44 pins while the Dell hard drive adapter inside the pliers has one hole in the middle blocked off.
This means I have to take a pin out of the CF adapter, good thing I only paid $1.57 for it...
Step 3: The Planning
You can see the missing pin on the old hard drive in this picture.
Step 4: The Fix
Step 5: Ready for Linux Install
The CF fits nicely where the hard drive used to sit, just ready to go.
Since the C400 cannot boot from USB, I pre-installed Slax Linux on the CF from my desktop to save me the trouble of burning a CD.
Step 6: It's Alive!!!
It now has been running since February this year (2013) without missing a beat, although It rebooted twice as we had severe thunderstorms, but it survived the power spikes.
Step 7: Conclusion
Best of all is that I still have my dinosaur Pentium 3 converted laptop that is still being useful.
Step 8: WAIT! Didn't You Buy TWO Adapters??
Oh yeah, here are the pictures for the D600 conversion, no real need to give more details other than I used the Instant Tac to secure the CF adapter to the hard drive caddy and to make sure it is positioned properly before inserting back into the laptop..
FYI, I did have to remove the extra pin for this conversion as well, so I wonder if spending a couple of extra dollars could have saved me this step, but it wasn't a big deal.
Initially I cloned the Slax Linux install from the C400, but once I made sure it worked properly I installed Lubuntu from a USB key. The new CF conversion shows as a regular hard drive so you can install any OS the laptop can handle as long as you have a CF with big enough room. With 8GB I can even store some favorite music on it.
BTW, as you can see from the last picture in this set you will notice that the D600 sits on top of a mini stereo system, this way I can also use the laptop to stream my music through it.
UPDATE - June 2014: The C400 is still running Slax, however the D600 is now running CrunchBang (32-bit Waldorf for Older PCs (non PAE))