How to Write-protect a section of portable hard disk?

No.

There is not way to guarantee no change if you give them the device.

The best bet is to only give them a copy.

You have to suck-it-up and buy another disc so that you retain the master copy, if you want to share info free of risk.

You can compare the returned disc to what you have. I recommend FreeFileSync.

As long as the hard drive in not in your hands there’s no way to be really safe.

If you are merely concerned about mistakes, where people understand they aren’t supposed to change those files, read-only will be fine.

But I agree with @ipso and @Thammuz, you should make a copy because you never know what might happen. The cost of a second disk is probably small when weighed against the time & $$ it would take to re-create the data.

Forget about “accidental deletions and moves” ... what if the disk never gets back to you at all? If it’s a complete collection of something that means a lot to you, you should definitely have a copy, whether you lend it out or not.

As for restricting access to “read only”, if you know what you’re doing then you can set up individual directories (or the whole disk) with Read-Only access. But even that won’t prevent someone from reformatting it or re-partitioning it. And it can always be lost or stolen.

Since 500 MB USB drives are going for around $100 or so, just get one (or more) of those and keep your copies safe.