Sandisk USB drive is read only [duplicate]

Sounds like the drive is just getting old and the flash has reached its write limits. According to SanDisk customer service, those drives have a safety mechanism that disables writes when error conditions are detected, to protect the data:

The flash drive has detected a potential fault and has become write protected to prevent data loss. There is no method to fix this. You will need to backup your data and replace the flash drive.

If you read the original quote further, I'm not sure what they mean by "looking into a solution to resolve the issue" (or why it involved "OS developers"...) as this is a good thing: when flash write cycle limits are reached, reads begin to fail as well... although perhaps they are referring to reports of premature / false fault conditions, or those drives are more sensitive than they could be, or the service rep just padded this useful gem of info with some canned customer support fluff. Who knows? But, I digress.

1.5 years is not particularly old but it's not new either, especially if you use it very frequently. Also, just from my own experience I've always found SanDisk flash (at least, their consumer-grade SD cards) to be reliable even in the worst situations; but 1.5 years is not a bad run.

In any case, sounds like it's time to back up your data and get a new thumb drive.