USB key/disk not identified as a storage device

I have a 64GB USB key that I suddenly can do nothing with...I've tried both Mac and Windows diagnostics, as described below. At this stage I don't mind losing its data (99% of which was recently backed up elsewhere); I would be happy just being able to reformat it and use it again!

All the below were done with root/Admin privileges...

1) The "testdisk" program on my Mac hangs while trying to find it (testdisk merely displayed "Please wait..." for hours overnight!). "Disk Utility" cannot see it at all.

2) The "testdisk" on my Windows machine doesn't hang, but cannot find it...it lists the other disks only.

3) On my Windows machine, the symptoms are exactly as posted by "mdroc1" here: http://ift.tt/1yQaEvF I can change its name in Disk Management , but it still shows as "No Media" (no partitions, nothing to format or recover). I cannot reformat it in DOS cmd: "Cannot open volume for direct access".

4) On my Mac, it won't mount, and "diskutil -list" doesn't show it [even though according to this webpage, it's supposed to show all connected disks whether mounted or not]: List all devices connected, lsblk for Mac OS X

...further down that page there's another command: "system_profiler SPUSBDataType". This returns loads of info; basically the "tree" of all the USB hubs and devices on the Mac. From that I see my "bad" USB key as:

               IS903:

                  Product ID: 0x0903
                  Vendor ID: 0x1f75  (Innostor Co., Ltd.)
                  Version: 0.01
                  Speed: Up to 480 Mb/sec
                  Manufacturer: Innostor Technology Corp.
                  Location ID: 0x1d113400 / 20
                  Current Available (mA): 500
                  Current Required (mA): 200

• 
  

  whereas a "good" disk like my 32GB USB key shows as:

  
  

  
                  USB DISK 2.0:
  
                    Product ID: 0x3e00
                    Vendor ID: 0x13fe  (Phison Electronics Corp.)
                    Version: 1.00
                    Serial Number: 07BC1B0B29D60838
                    Speed: Up to 480 Mb/sec
                    Manufacturer: 
                    Location ID: 0x1d113330 / 21
                    Current Available (mA): 500
                    Current Required (mA): 200
                    Capacity: 32.03 GB (32,027,705,344 bytes)
                    Removable Media: Yes
                    Detachable Drive: Yes
                    BSD Name: disk15
                    Partition Map Type: MBR (Master Boot Record)
                    S.M.A.R.T. status: Not Supported
                    Volumes:
                      DANE_32GB:
                        Capacity: 32.03 GB (32,027,704,320 bytes)
                        Available: 6.21 GB (6,210,011,136 bytes)
                        Writable: Yes
                        File System: MS-DOS FAT32
                        BSD Name: disk15s1
                        Mount Point: /Volumes/DANE_32GB
                        Content: DOS_FAT_32
                        Volume UUID: F7F9D1DF-5081-3D18-8CE1-D8913FA2F438
  
  

From all this, I infer that essentially, it's still a working "alive" USB device, but no computer I connect it to can tell that it's a storage device. The kind of detail that "system_profiler" shows for it is the same as it shows for a dumb USB hub. Since the computer doesn't realise it is a disk, then there is just no handle on it for (re)formatting.

Any suggestions, please?