Roanantelope :
Random Stalker, you are a star. If I change over the position of the main drive and leave the slave disconnected the system runs up perfectly. If I replace the slave plug the fault re-occurs. If you could explain about checking the drive mode in bios, I would be most grateful. If you don't hear from me tonight I will be on line first thing tomorrow. This is one of our very important computers as it is used to drive the security x-ray machines. Roanantelope.
Well, since you're using ATA drives, it will not be applicable to you. But since every question deserves an answer - the BIOS drive mode lets you to switch between two modes of communication for the newer SATA hard drives - AHCI (an Intel made standard which supports greater speeds and many newer functions like TRIM which is vital for SSD at the cost of compatibility) and IDE (which is a WD standard supported by any OS which lacks any newer feature)...
Now back to your hard drives. Since some play well with each other and some not - sometimes it helps when you set one drive to cable select (CS) and other to slave (the CS master drive goes at the end of the cable, slave in the middle). Sometimes it works other way (CS as slave, master as master). Two CS don't like each other, but you're welcome to try...
Also, some hard drives love to be master and won't work other way (had a maxtor drive like that).
But if everything else fails, you can always connect the data HDD to the second IDE cable (even if it's not recommended to put a drive on the same cable as the CD ROM, but, if it helps...)