How do I delete files permanently from a usb flash key? On my mac (Yosemite 10.10) it's telling me the files are deleted, however on plugging key into television they're still there

Question

I've a problem I've never come across before and it's how to permanently delete files on a usb flash key? I've just migrated from using Windows for 15 years onto Apple iMac and had been deleting files easily and permanently from the key on Windows without any hassle and thought I'd done the same on my iMac but seemingly they're still there as the tele is showing them when key is plugged in, BUT they can not be seen on iMac. I wondered why the key wouldn't take anymore files after, what I thought was, deletion and now I know why-imac hasn't deleted them!
I've actually gone into Disk Utility and formatted the key in it's usual FAT32 format to finally delete the files.  Anyone any ideas as to why this happens and how to solve this please?
Many thanks
puretoon

Answer 1

In addition to what Drew wrote, remember the Mac has a data fork and a resource fork.
Here is an old thread but a nice recap by Dr. Smoke
._Files
Note that the ._file files you note are the resource forks of Mac files you copied to the drive from your Mac, and these are only visible when you use the Flash drive on a PC. These are normal and indicate the Flash drive is already formatted for Windows.
When you copy or save a file from a Mac OS X system to a Windows shared volume or a Windows-formatted disk, the Mac creates two files: the data fork (xxx) and the resource fork (._xxx). This is called Apple Double Format and is normal. The resource fork contains additional data about the file that is exclusive to the Mac that cannot be saved otherwise on Windows-formatted disks. This is new with Mac OS X, as documented in Mac OS X: Apple Double Format Creates File Name With the Prefix '._'.
The resource fork files (._xxx) are necessary if you want to copy the file back to the Mac and retain the metadata. The resource forks of files are invisible when they are used on the Mac: the Mac OS Extended file system (aka HFS Plus or HFS+) uses forked files, with a file having both data and resource forks.
You can delete the resource forks on the Windows disk if and only if you don't care about losing the resource fork information should you copy them back to the Mac, such as comments, image preview icon, etc. Some Mac applications use the resource fork data, so deleting the resource fork can be problematic if you copy the file back to the Mac. Likewise moving the file while not also moving its associated resource fork when using the data on a PC can result in problems when using some files again on the Mac.
You can't prevent the resource forks from being created if you copy or save files from the Mac to the Windows share or Windows-formatted disk via Finder.