you will have to format the hard drive with FAT32 file system before WinMe can be installed. FDISK is a utitlity that can create, delete or format hard drive partitions. either make a bootable floppy and put the FDISK utility on it, or just use a WinMe boot disk which are already setup with FDISK.
you can download a WinMe boot disk from here:
http://www.bootdisk.com/bootdisk.htm
using FDISK will completely wipe any data stored on the hard disk but as you say uv already wiped it, this shouldnt be a problem for you.
the link below will guide through using FDISK:
http://www.blackviper.com/Articles/OS/fdisk/fdisk1.htm
EDIT: you say this is a "backup disk", are you sure this disk has the full WinMe OS on it?