Stolen from GRCC on the neo forums, the guy is a legend.http://www.xboxstorageupgrade.com/hh/web/home/hhauthentication.jsp
Instructions are all in there, you just need to make sure the site is set to the right country (top right). Free 512MB memory cards aren't to be sniffed at and 20GB HDDs for £12.99 certainly aren't to be sniffed at. The HDDs are refurbished rather than brand new but considering a similer pre-owned model will set you back around £50+ it's a damn good price all told.
Microsoft are giving them away so everyone has enough room to download the NXE in November. I'm not complaining!
Or that one if you're still suspicious.
Just to clarify who this offer is for:
- Xbox 360 Core Owners (The original non-hard drive model). If you only have a 64/256Mb Memory Card for the console. If you have since purchased a Hard Drive the site may flag this and not offer you a HDD/Memory Card. If you are eligible you will be offered either a 512Mb card for free, or a 20Gb HDD for £12.99
- Xbox 360 Arcade Owners (The current non-hard drive model). If you only have the 256Mb Memory Card and have not purchased a HDD yourself then you should be eligible for the £19.99 20Gb HDD/3 Months Xbox Live pack. The 3 Months of Xbox Live Gold has an RRP of £15, effective value of about £6-10.
If you have a HDD model of 360 (20Gb/Pro/Elite), then you're not eligible (as you don't need extra storage space).
Microsoft checks their device database not only for the ID of your console (to work out which model you have), but they can also check if you have a HDD or 512Mb Memory Card (based on what devices they were originally registered to). If all else fails and you're certain you're owed something, give MS a call, it can't do any harm (although give it a couple of days as I bet Xbox Support is getting swamped at the moment)