SSD Dashboard not recognising drive after firmware update failure - HELP


I have a SanDisk Ultra II 960GB SSD. 

It’s only been lightly used as a boot drive and I’ve just actually sold it to someone as I need money (was going to post it to the buyer today)

I just popped it in a USB caddy and went to SSD dashboard to secure erase it. Happened to notice there was a prompt for a firmware update - tried to update the firmware and got an error message (which I didn’t write down) saying the update had failed. 

It initially seemed to still be recognised so I went through the process fo secure erasing, it said this had completed then immediately after this it greyed out lots of options in the SSD dashboard and said there was no SanDisk SSD present. I presume this is to do with the firmware now being corrupt?

I went into Windows and it can still see the drive there and it is clear it has not been erased at all - has all my files and folders still present. 

What can I do now? Even if I find another way of erasing it I can’t really send to the buyer in this condition since they are likely to attempt to use SSD Dashboard and will encounter the same issue. 

Can anyone help? This is fairly urgent as I need to post this out by tomorrow at the latest.