Windows 7 has driver issues, it will try to install drivers for the cradle/drive and fail (you'll see under Device Manager, there's an exclamation against either a Disk Drive entry, or a USB Mass Storage Device).
If you un-install the driver for the affected entry, the device will be re-detected and suddenly magicly start working as you'd expect.
On other occasions, it can take over an hour (based on my 250GB test drive) for the DMS to be recognised - as I think the O/S does a complete scan of the disk for some reason.
If on Windows XP, when you plug the DMS into the cradle, the PHTSYS and PHTDTA partitions are not showing up first try the cradle in another USB port. If it's still not showing up, then there may be two likely reasons:
1) The switch that activates the power to the cradle may be damaged. If you look into the cradle, one one side of the connector the DMS plugs into there is a switch. It's possible you are either not getting a good connection with this switch - or it's in someway faulty. If you are good with electronics, or know someone that is ... you may be able to repair this or swap this bit out entirely as others have done in the past
2) There may be a fault with the cradle. When you plug the DMS into the cradle do you see the red/green light illuminate at any point?