As Eileen is trying to tell the OP, formatting the hard disk using the keyboard's format function will effectively clear the hard disk and will be the simplest option.
There is just one thing to check first! If the T5 is fitted with an expansion memory board, and it currently contains samples for expansion voices, the hard disk will contain a folder called 'Expansion' which contains the voice data for these samples. If you delete the Expansion folder, it won't delete any samples in the expansion memory, but it will render any voices that use the samples inaccessible!
If there is no expansion memory fitted, or there are currently no expansion voices in the Expansion folder on the hard disk, then you can go ahead and format it. If there are such voices you must decide if you want to keep them or not, and if you do, then you will need to manually delete everything on the hard disk except for this folder.
Note that this format is effectively the same as a 'quick format' on a computer, since all it does is clear the FAT table which tells the controller what files are stored on the disk and where they are.
After this type of format the actual data is still left on the hard disk (and could be recovered assuming nothing has been written to the disk since the format). However the disk will appear empty to you (and the T5 OS), and all of it will be available for use. As you use the disk any previous data will simply be overwritten.
If you decide to format the hard disk, you access the display containing the format function by pressing [FUNCTION] > UTILITY [H button] > Tab to [MEDIA]
Regards
Ian