I thought I'd chime in that there's some great tools, like the regular parted, that you can use in addition to the GUI tools in Slax (or other distros as well).
"Dunno why it couldn't be formatted directly from Slax. "
For anyone less than an "expert" , I think it is still best /safest to use a gui drive solution to avoid errors such as formatting the wrong partition , especially if you have more than one media mounted , and numerous partitions.
Judging by OP's question they want to nuke the entire drive:
i need to format my HDD
Still more info is needed. What do you intend to do AFTER you format the HDD? Install windows? Install slax? Install some other linux flavour? use it as storage?
umount /dev/sda1
mkdosfs /dev/sda1
Be warned you will lose everything that once existed on this first partition. If this is not the desired outcome, then go with a GUI boot disk type thing.