The status of the disk is that it has no drive letter - it was J before I installed XP Pro. During installation I just single partitioned the OS drive.
At re-start I noted that J drive had not 'loaded' everything else had quite normally. In disk management the drive is third as it was intended to be but is now 'dynamic' and 'foreign' and is disk 2 without a drive letter.
I have tried disconnecting it and re-connecting onto the IDE controller together with a CD-ROM, the BIOS sees both OK - as it always has.
The conversion back to basic seems to be the answer I am told but like I say I would like to hear from someone who has done it.
Don't know if that helps SD. Thanks so far for your help peeps.