Por si a alguien le sirve lo he arreglado así:
Installing a Secondary (Slave) Drive:
Note to users who do not want to format an old drive that will be used as a slave drive: To add a secondary drive from another system (old PC) without reformatting, the drive must have an existing FAT16, FAT32, or NTFS partition. The drive must also be jumpered as a slave drive. Add the drive to the system and Windows will detect the drive.
Insert the MaxBlast CD or Floppy and restart the system.
When MaxBlast asks if you would like to prepare the drive for use on your system, click no.
Select "Set up Your Hard Disk".
A list will appear with all the drives on the system. Click on the drive that needs to be formatted and click on "Next".
Choose the operating system on the Master "Boot" drive.
Choose “Install drive as additional storage”
Choose an installation method and follow the onscreen prompts. "Easy Installation" will create one large partition on the drive. "Advanced Installation" will give you the option to create more than one partition.
Proceed with the rest of the instructions.
Remove the floppy or CD when MaxBlast is finished. Restart the system.
The new drive is now ready for use.