A question for those well informed in formatting: when reformatting a hard drive, are there bits that don't get formatted? Such as the MBR? And are there programs that can write/ read areas not usually read or written? I only ask because AutoCAD seems to have written something somewhere that survived a reformat with "warning all data will be lost" warning.

Formatting places a file system on a partition, and as such does not wipe the entire drive. MBR is one location not affected by formatting. If there was data on a partition before formatting, much of that data may still remain. The file system is made up of some records that tells the OS where to fine the records that belong to a file.

There are programs that will overwrite every bit, on the disk surface, but not a format.

Normally after format the data still keep inside the harddisk (normaly we just format the index of the hard disk).

Regarding the autocad issue, I think it is a CAD tool and no relationship with your harddisk.
Please confirm.

I think "formatting" often means setting up the filesystem on a partition --not the whole drive. In other words, if you started with an empty drive, you would first create partitions and then format them for a particular filesystem.
For example--in Windows--if you format the "C Drive", you are really formatting the partition which Windows calls "C:"

I have seen various occassions where programs were reported to write things to the raw disk (ie not in a partition). The only way to get rid of that kind of thing is to "wipe" the whole disk.

After reformatting, and reinstalling AutoCAD, the installation knew there was a version installed before. I was surprised, simply because I thought a reformat wipe the slate clean, so to speak. Even if the data were still there, how, in the absence of the previous index of the file system, did the installer find the info? It must have known exactly where to look, in which part of which cylinder. Or it wrote something somewhere that is not affected by a reformat!

Some programs use the area between MBR and first partition for copy-protection stuff. Format cleans up only the partition you are formatting, other partitions and unused areas on your HDD are not touched.

Can anyone suggest a wipe-out-everything-on-the-disc program? Don't want things left there. Something that writes random 0 and 1 to all parts of the disc?

If your trial period expired I suggest you buy the program.

You might try something like DBAN

Thanks for the help, very pleasant. Can't afford AutoCAD, I live in hope of finding a Linux version. But once, whilst looking for a Linux version, which was valued at $ 180 US, I noticed that the internet address was, no joke, bankhackers.com. I felt a bit reluctant to purchase, as you might understand!
I use Fedora nowadays, when I can!

Quick Google shows that there is no AutoCAD for Linux---but there appear to be some "work-alikes".

Have you searched for "Linux CAD"?