Newly purchased used Dell/EMC AX100

I've just purchased a used AX100 and I can't seem to find any technical white papers on the net for this unit

This EMC link has some info and a way to download software (assuming the unit is still under Dell warranty): http://www.emc.com/dell-ax100support

How easy is it to reset the unit to factory settings?

It's possible, but requires EMC engineering level access (way beyond the engineering level access that a lot of people may be familiar with). You can use the initialization utility to find the IP addresses of the storage processors, and to reset the password, use the information at the link I provided.

I've tried connecting via the RJ45 serial but get nothing but jibberish on hyperterminal, settings used are 9600, 8, n, 1, flow control = none, ANSIW emulation, I haven't got the Dell/EMC serial console cable but I am using a cable that I know works fine,

EMC doesn't use regular pin-outs for the DB9-RJ45 connection, so a Cisco cable will not work here. You have to get an EMC (or Dell|EMC) cable for this.

The unit currently has 250Gb's installed and I want to replace with 2TB SATA drives, anybody done this?

You are in luck with the AX100-series; it's the only EMC system with SATA drive support that doesn't have a 'drive lock' in the software. The successors to the AX100 (AX150 and AX4-5) have a software lock to prevent you from using non-EMC drives on the systems (they often aren't even compatible with eachother (e.g. AX100 certified drives will not work in an AX150 and vice versa)). The drives may show an "unsupported" when you're in Navisphere, but they should work.

First I'd suggest to verify that the fault light on the storage processors is off on both storage processors. This would indicate that the controllers booted just fine. When powering up the system, the fault lights will go through the following stages:

- slow blink (once every 4 seconds): bios test

- medium blink (once a second): POST

- fast blink (4 times a second): OS boot

If the led blinking keeps cycling or gets stuck on the fast blinking, you would need the proper serial cable to figure out what is happening. One possibility is that none of the first 4 drives have a valid operating system section on them for the array to boot off of. If that's the case, you have a doorstop pretty much as there's no way (outside of access to the factory build system) to load the OS on the system. If the core is there, but the OS image is corrupted, you can use the link I posted to find info how to restore the OS (may require access to the downloads, which may require a valid warranty).

Hopefully this helps some.

Any clues on what drives people have found that work? 1TB, any specific model?