Hi,

Could someone tell me, is it the jumper positions that decide which drive will be master / slave, or is it the position you PLUG them into the cable?

My PC works ok with the current master/slave setup, but if I change the JUMPERS so the drives change roles, I just get to the WINDOWS XP loading page, and it NEVER gets any further. Why is this please?

Kop442000Both.
40-pin connector/40-conductor ribbon cable - Master/Slave is the proper jumper configuration.

40-pin connector/80-conductor ribbon cable - Cable Select is the preferred jumper configuration.  
Blue connector - System Board
Black connector - Master
Gray connector - Slave

http://www.mikeshardware.com/howtos/howto_connect_ide_hd.html
you can try to set both HARD drive jumpers to cable select, then on the IDE cables, the very end should be connected to the master hard drive and the connector in the middle of the cable should be connected to the slave drivelook just connect both of your hard drives on the primary IDE Controller, and jumper your second hard drive on slave.....THATS all it should take