When your external USB disk or drive fails to be formatted by Windows system tools, next step is to hire vendor-specific tools, that usually ships to you along with that drive. If this also won't help, then low-level format seems to be the only option. Here you'll find some tips on how to do this under Windows.
A small, yet important notice for the begining. This quick guide is not for idiots. I assume, you're a clever man, so I don't have to warn you that formatting any drive with low-level method will destroy all your data in the way, on unformat, undelete or partition recovery software can handle. You may spend two or three thousands bucks for services of company such us Ontrack, and they may be able to recover your low-level formatted data. As they are able probably to recover data written to a wooden stick! :] But in general, any drive treated with low-level formatting procedure is considered to be unrecovable at all. And most times even prayers won't help.
You have been warned!
So, what's about low-level formatting? Well... Most drive formatting techniques does not actually physically format disk surface. It only tells the system (by modyfing partition table), that the data is erased (formatted). This is much faster (full formatting of huge disks can takes hours) and allows to recover any data from such "formatted" drive, as long as you don't wipe it or save new data over old one.
If you choose to wipe-out partition instead of formatting it, a piece of software will write zeros, ones or a random values to each sector of your disk. Data from such drives is unrecovable for most software (see above notice on Ontrack magicians), but it still doesn't actually means physical drive format process in most cases. And since this is still done via software methods, you're most likely be halted by Windows, when you try to format system drive or do something simillary stupid.
Now, low-level formatting is a physical method of formatting drives. It causes each sector to be physically reset, by putting a small charge through it. This could pottentialy cause a hardware errors on your drive or make it completely unusable. For these reasons, first versions of low-level formatting software was included only within BIOS of a computer and was advised to be executed only on drives already marked as broken. It was like 50/50 chance. If your disk had bad sectors, partial physical damage or some other unrecovable errors, you could treat it with low-level format and you would ever get freshly new, fully working drive (all bad sectors reset) or you would kill it permanently. Fully working, means that sectors marked as bad were reset. Because, if you had real physical damage to your drive surface, like after combining your hard drive and screwdriver together, not even low-format, but also prayer wouldn't help.
Times has changed. Low-level formatting tools has been available even for Windows and you can do pretty much harm to your drives even for USB ones.
HDD Low Level Format Tool 2.36 is said to be able to low-level format under your Windows. It is the only thing, which I have found under "low-level usb format" search and it has many positive opinions. Many people says that it worked for them and has fixed drive that was unfixable even by software disk manufacturer provided. Unfortunatelly, it failed for my disk, but this could be because that one was really badly damaged. Anyway, running this software for my disk ended up with issuing many error messages like "Format Error occured at offset...". After which the only partition table on that drive was deleted and disk remained with unallocated space on entire surface. No actual low-level formatting took place and no bad-sectors were reset. Pity.
At the end I can only repeat my warning. This is not a toy. Many people said that it worked for them and you can assume that drive treated with this or similar software will really be unrecovable at all. Take it with extreme caution, especially that this software claims, it can also low-level format your system drive. I wasn't that stupid to check, what happens, if I do that. So I don't know, if Windows could block such attempt and save your system disk from your extreme stupiditty. But low-level formatting was always considered a hardware, direct disk access method of formatting drives. So, I wouldn't be much suprised, if it would turn out that with this small program you can actually low-level format your master disk and say bye, bye to Windows and your data for long time (for ever, you're geek enough to not have any backups). Have a nice day! :]