I use Virtualbox for the very few occasions where I can't do something in Linux, but I underestimated the size of the .vdi file I would need and am now short of space. Most solutions to this problem start with the words 'create a new vdi file of the size you want' . This makes one fundamental assumption which is you have room to do that, well I don't. The only way I can create space for a new vdi file of the size I want is to delete the old one and that is not going to help me boot the vm.

I thought I had found a solution which was to create a small second virtual drive and attach that to the vm as a slave drive to give me the extra space I need. This worked perfectly apart from one small flaw. Windows does not recognise the second hard drive. I thought it would turn up as Drive D in 'My Computer' but it doesn't appear at all. If I hover my mouse over the hard drive icon in virtualbox it clearly shows that I have 2 drives attached, but windows only sees 1. The add hardware wizard doesn't help because as far as 'Device Manager' is concerned both drives exist.

What do I have to do to make the second drive appear in 'My Computer'?

Did you go into windows volume manager to format the new virtual drive?
By default it has no filesystem (raw) and should be formatted as ntfs or whatever- else windows can't assign a drive letter to it, of course.

What a fool I am - of course that is the answer!!
Thanks scarecrow, just a few seconds work and now I have a second hard drive to use without deleting the first, just what I wanted.