How do I know this, you ask?
Because I've just had the pleasure of putting a second drive in my PC, courtesy of bohemiancoast, and the only thing I can assume, after wrestling with the bastard for about 30 minutes to get the job done, is that the guy who designed it assumed that anyone putting a drive in would have X-ray vision and super-strong fingers. It's impossible to put the second hard disk in with the cables attached, and you can't attach the cables without putting in the primary hard disk - at which point you can't actually see the sockets for the cables, and have no room to exert any strength to push them in.
Anyway.... Thanks to a very generous gift my PC is now nearly doubled in capacity - I've pulled the old second drive, which was 40gb, and put in the new 110gb drive, which I'm now formatting. There's another 40gb on the primary drive - which I think will stay as primary for now. Next step is to bung the old drive into my Windows 98 box, roughly octupling(?) its capacity since the current drive is only 6 gb. But I need to check if the old machine can read it first - there's nothing immediately important on there, but I do want to keep a few of the files so I can't format it. This may be my cue to buy one of those "make your hard disk into a USB drive" kits, or at least borrow it for a few days.
Thanks again to bohemiancoast - hopefully I'll be able to return the favour sooner or later.
 Also an HP box, but designed a little more sanely - possibly before Superman joined the company...
Later: And of course I'd forgotten that the last time I switched the Windows 98 box on, a few weeks ago, was to put in a spare 20gb drive. So I'll be upgrading from 26 to 60 which is not quite so dramatic. Snag is that this means I need to move the operating system etc. from the 6gb drive to the 20, then ditch the 6 and put in the 40 as the secondary drive. Anyone able to suggest some reasonably idiot-proof software to do this?
And a little more later: Someone on ljgenie tells me [incorrectly, as it turns out] that all I need do is sys D, copy all files across, and away I go. Sounds good - let's give it a try. But I will need that USB kit, Windows 98 doesn't know about NTFS drives. High time I bought one anyway.