I updated my computer to macOS Big Sur, and now I can’t write to my external hard drives, even though I have the correct permission settings. I can still read from them, but any attempts to copy files to them fail. They are formatted as Mac OS Extended. I’ve checked the “Get Info” option and ensured that all accounts are set to “Read & Write” and that “Ignore ownership on this volume” is enabled. However, it still says, “You can only read.” One thing I noticed is that a software called Cylance appeared randomly on my Mac, which may be related to this issue.
Is this a company Mac? If it is, you should ask the IT department for help. The problem is obviously not about the permission settings. My guess is that your Mac is restricted by certain software to write to external drives for data security. Chances are that you can put your external drives on a white list to regain write access.