Thumb drive not recognized after reformat attempt

SEB575 wrote:

Here is the problem...it is not recognized...says UNMOUNTED...yet It is connected to the same Apple connector that it has been connected to since the get-go...plus the exact same thing happened to TWO different USB Thumb drives. Mounts correctly, readable, start the erase/reformat process thru Disk Utility and zap...gone... two different brands of TD...very odd

Linc Davis makes a point— "Some flash drives have a failure mode in which they are irreversibly write-locked by the internal controller when it detects a write error. If you can't erase the drive in Disk Utility, that's almost certainly what has happened. To be sure, test it with another computer, if possible."

SEB575 wrote:

I had a thumb drive from a vendor that my MacBook Pro recognized. I wanted to erase and re-format for personal use, When I did that thru disk utility, it ejected the disk and when I plugged it in again the error message said "The Disk you inserted was not readable by this computer." Any ideas?

Erase and reformat a storage device in Disk Utility on Mac

https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/19.0/mac/10.15

SEB575 wrote:

I appreciate the link, but that is what I already did.

When I renamed it and chose ExFat (I sometime use a PC) I clicked erase in Disk Utility and POOF! It unmounted and then was not recognizable when attempted to remount....

"I had a thumb drive from a vendor..."

I have had luck with problematic USB formatting using a camera to do the reformatting—then it seemed to work.

On second thought that was SD card and cameras.

USBthumb drives are cheap, often not so reliable and who can speak to some " thumb drive from a vendor"

Your alternatives are either GUID Partition Map with a exfat file system or apfs

or

Master Boot record with exfat

If you are using a USBC to USB adapter verify you have the thumb drive shoved in far enough for all pins to connect.

SEB575 wrote:

One was a large capacity thumb drive, so I wanted to check possibilities before chucking them.

Well sure—

try reformatting on an alternate Mac or better yet a PC and see if this makes the difference.