@Butters
Certainly rustybin's problem is quite unusual as you pointed out. We'll give him a chance to reply to the above post and then I'll move all relevant posts to a dedicated discussion thread because there's some very interesting and particular facts eventually uncovered here. Flash damage can have many guises and this certainly looked just like a simple soft-bricking until it became obvious there was a flakiness of preboot but card reading was available .... a VERY unusual presentation.

@rustybin
I encourage you to try what Butters suggests above. Check for bad blocks then if there's very few of them re-formatting as suggested, initially using the LLF Tool and then RMPrepUSB.

Please report back in any case. thanks.