A Format of a hard drive may only replace the cluster markers, leaving files recoverable (sometimes one cluster at a time, which is tedious).

Even overwriting with new files may leave "recoverable" bits of the old files, unless sufficient multiple overwrites are used to "bury" the original bits.

Back when I was forced to deal with "security issues" a.la. government regulations, the preferred utility was a Norton Utilities "Disk Wipe" that included an optional "Tempest Secure" routine that:

1. Wrote all zeros to every empty bit-space on on the drive.
2. Wrote all ones to every one of the same bit-spaces on the drive
3. Repeated steps 1 and 2 SIX TIMES.

There probably are numerous utility programs available now that will do a similar "secure delete" but I don't have one to recommend.

Many hard drive (and other equipment salvage/recycling operators) now physically shred hard drives. Your first option is to ASK THE MANUFACTURER what their policy is on recycling of returned drives. IF THEY HAVE a published policy that looks safe enough, you probably can trust them to not look at your data, or to allow it to be "passed on" to someone who might attempt to look at it.

If you really need to be sure of "secure wiping" your simplest option would be to get a utility that offers what you want.

You should be aware that for older hard drive formats, through FAT32, there is a specific limit on the number of files that can be written to the root folder of a drive, so "amateur" attempts to write a full disk may fail. For FAT12 or FAT16, used on older floppies, the root folder can contain only about 256 or 512 files (and folder is a file. For older hard drives, the number may be more like 1032 or 2064, since the root sector of the file must record the "starting cluster" for each file, and there are a limited number of bits to record these "first cluster locations" - and hence a limited number of files that can be written. For NTFS format, the limit is "very large" depending on free space available to expand the "table."

A folder on the drive can contain any number of files, so you need to delete all, then create ONE FOLDER in which to "write over everything" with a bunch of junk. And then do the write-over multiple times - including deleting the folder and making a new one - with a different bunch of junk each time.

Even this is not totally "secure" in all cases, since you probably will need to leave the OS in place and some data - and data recovery info - may be contained in "System Files" within the OS. (Since you're indicating a USB drive, there shouldn't be any critical System Files on the drive you want to wipe, so this shouldn't be a problem in your case. Note though that the "recovery info" you're seeing may be coming from your System Hard Drive and not necessarily from the external USB drive, depending on your OS.)

Manual wiping can be extremely tedious, especially with larger drives, without a suitable utility.

ASK THE VENDOR who will get the failed drive about recyling policies first. If their answer is not satisfactory, then get a good utility that offers "secure disk wipe."

John