watch out that ubuntu and slackware have different initialization on harddrive.
ubuntu reads it as hdx and slackware as sdx
so if u use grub and u want your slackware in your boot menu u have make different initiation.
and put your slackware like these :
kernel /boot/vmlinuz root=/dev/sda5 ro
see in first initiation the root said hd0 (this is ubuntu command)
than after that the kernel use different name in root=/dev/sda5
and wala they worked