If "disk" happens to mean "solid state disk", you are somewhat lucky. You can instantly make retrieval of data (almost) impossible by using the manufacturer's secure erase tool. This process is both very fast and secure due to the way these drives work: They encrypt all data that is written to the drive, no exception made. Doing a "security erase" simply tosses the key overboard, and the data, while still present, is unrecoverable without the key.
Otherwise (i.e. "disk" means "harddisk"), the only safe method of making data unrecoverable is by disassembling the device and physically destroying the platters.
The reason for that is two-fold. First, overwritten data can still be read. Data density has admittedly gone up considerably during the last decade, and some people argue that what's overwritten once is basically unrecoverable, but if it is really, really important, I wouldn't bet my right hand onto that. I haven't tried to recover erased data during the last 15 years, but it used to be that recovering 5-6 times overwritten data was kinda tedious, but otherwise absolutely no problem. Maybe that's different now, maybe you only need to overwrite twice, or maybe once. But there's a bigger problem ahead.
The second, and much more important issue is that you don't know if and when you overwrite something. This is a very serious problem.
Modern drives do transparent wear-levelling and sector reallocation, and maybe even caching on MCL (hybrid drives). You have absolutely no knowledge, or control, what gets written when and where, or what gets overwritten.
Thus, even a "secure erase" that overwrites the same file ten times with different patterns might indeed overwrite ten different sectors on the disk, none of them belonging to the original file (and you might possibly still have an old copy of the data stored in a retired, now inaccessible block).
The only way of knowing for sure is disassembling the drive, putting a neodym magnet onto the platters for a few minutes, and giving the board and platters a little treatment with the hammer afterwards.