I'm suprised that you're having snags like this with mandrake 10. It's very uptodate (with only a few updates to install after), so if I was in your shoes, I'd probably be trying knoppix as well.
their hardware detection is second to none (IMO) also, if you find that it boot's OK, you can always do a hard disc install, and you can find more about that at the knoppix forum's
(or if you prefer it in German, there's a different URL).
I should mention that knoppix on the hard drive is great, it's debian based so you've got shit loads of app's/packages available. You'd probably find all the usual suspect's are installed by default.
In truth, I can't see anything that stand's out as to why the disc's won't install OK (maybe something with the hard drives i.e. if they're SATA, but that still shouldn't present to much of a snag, or is you've got them set up as some sort of raid array, but again the same).
Just a quick pointer though, you may need to get the hard drives partitioned/formatted first (especially if you're trying to install gentoo), though in theory, at least, mandrake should normally be able to handle that (though I'd point out that I've done all my partitioning with Partition Magic 8).
The gentoo install instructions are excellent, well explained, and pretty clear (which is saying something for linux doc's). But don't forget, you should opt for the stage 3 + GRP install if you're going to try gentoo, because if you don't and you just get the first disc, then when it comes to installing a window manager (kde for example, and gnome for that matter) then the download doesn't take too long, but the compile takes forever. By this, I mean that when I first installed gentoo, I got through the first disc part of stage 3 (as this was suggested to be the quickest, it took about 2 hours), I couldn't work out how to get the packages off the second disc and just did
the kde packages totalled about 180 meg's, which should have taken me about an hour and a half of download at "half a meg", but the compile took about 15 hours, it get 1 package then compiles it, the moves on to the next part in the "portage tree".
So that's why I say use the "GRP" bit, that's pre compiled packages, you can always update things later, and that's gotta be quicker than 15 hours of compile.
What I will definitely say is that the doc's should be printed off, it makes life so much easier to follow (if you can't find a link to a printed version, change the URL. Instead of the .xml that it opens as in your browser, change it to .xml?style=printable). You will probably need the basic installation doc's and the desktop setup guide to start with.
Once installed it's very very easy to manage and keep up to date. Plus there's absolutely load's of stuff/assistance at the gentoo forums
though at times they do expect some prior knowledge - suck it and see!
Good luck (and follow gentoo's default partitioning scheme i.e. /boot /swap and /root and you shouldn't normally have a problem).