If I'd installed the disc today, that could be the answer Steven - but the new disc (which is the secondary one) has been in place since the beginning of the year with no problems, and I haven't done anything to change the jumpers which were set when it was installed, so I don't think that could be the problem.

I pushed the key for acessing SETUP, and went on to this screen more or less:

Standard CMOS SETUP

Hard disks       type    size      cyls   head   precomp   lands   sector mode
Primary master   user 10200 1240 255         0       19772       63      LBA
Primary slave         0         0    0          0               0                0       ----         
Secondary Master user 40018 4865    255      0    1957             63      LBA    
secondary slave - 0 (etc)                                                             Auto

and below there are ihe instructions for navigating this screen, and changing the bits in italics. Looking at it, it measn very little to me - I wondered whether there is some problem about it being Primary Master and Secondary Master, and one of them should be a slave? And did I put the right connection back - there are two identical ones (and I've tried both to no effect)

But perhaps the wise ones of the Cat can enlighten me?