Since it is a
Macintosh Plus, the hard drive would be separate. Which operating system is installed right now?
Do you have access to another older Macintosh computer with a built-in floppy drive?
Generally speaking, one can use Apple HD SC Setup 7.3.5 (
here) or
Drive Setup 1.7.3 to low-level format (initialise) a SCSI hard disk. Drive Setup (Zero all data under Initialization Options) can overwrite with zeros. A special utility such as Wipe Info (part of the commercial Norton Utilities package), or the freeware program Burn 2.5 (try a Google search), will be able to carry out additional overwriting operations. Depending upon the system version et cetera, the utilities may or may not work as intended.
Jan