If it's on the primary cable, it will be /dev/hdb . If it's on the secondary cable, it will be /dev/hdd .
It shouldn't cause any trouble booting. Once you're up and running, you can see what partitions are on the drive with "fdisk -l /dev/hdb" as root. Use hdd if you need. It will list something like...
Disk /dev/hda: 60.0 GB, 60011642880 bytes
255 heads, 63 sectors/track, 7296 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/hda1 * 1 16 128488+ 83 Linux
/dev/hda2 17 49 265072+ 82 Linux swap
/dev/hda3 50 1337 10345860 83 Linux
/dev/hda4 1338 7296 47865667+ 83 Linux
Then, try mounting a partition and seeing what's on it. As root, "mount -t ext3 /dev/hdb1 /mnt/floppy" then "ls /mnt/floppy". If it doesn't list the files you want, unmount it with "umount /dev/hdb1". Note, there isn't an N betwixt the U and the M in umount. If it gives you an error about the filesystem type, try ext2, or reiserfs, or whatever-it-is if you can remember from when you installed. I don't know what Mandrake defaults to, but it's usually ext3 or 2. If the files aren't there, and there's another partition, try mounting that one. You can skip "Linux Swap".