How do you back up a tag before overwriting it?
Read the tag with a tool that can export what it finds, and save that export somewhere durable before writing anything. There is no undo, no version history and no recovery once content is overwritten, so the read you take beforehand is the only copy that will ever exist.
Why it matters
Writing replaces content with no trace of what was there. Nothing keeps a previous version, no tool recovers overwritten bytes, and the tag itself has no memory of its own history.
So the moment before a write is the last moment the old content exists anywhere. If it mattered, it has to be captured then.
What a backup captures
The content: the records, their types and their payloads. That is what a reading tool exports, and it is what you would need to restore the tag or to write the same thing elsewhere.
The technical details are worth capturing alongside it, even though they are not restorable: the chip, the tag type, the identifier and the free space. Those tell you later what kind of tag it was, which matters when sourcing a replacement.
What it does not capture
Anything outside the content region, which is more than people expect.
Configuration settings, passwords, mirroring, counters and lock state are not part of an export and are not restored by writing the content back. A restored tag is a tag with the same content and a different configuration.
Data written outside the NDEF region by your own software is also invisible to a normal export. If a deployment uses raw memory, backing it up needs a tool that reads raw blocks.
When to bother
Always, for a tag you did not write yourself. You cannot know whether the content matters to someone else, and picking up an unfamiliar tag and writing to it to see if it works is how deployments get quietly broken.
Always, before rewriting a tag that is part of a working system. The content is the documentation of what that tag did, and it is frequently the only documentation.
Rarely, for a tag you wrote five minutes ago from content you still have. The backup adds nothing you do not already possess.
Back up one, not all
Tags from the same batch hold the same thing, so one export represents them all. What is worth recording per tag is the identifier and the location, not a separate copy of identical content.
Frequently asked questions
Can I restore a tag from a backup?
You can write the same content back, which restores what the tag says. Configuration, password and lock state are not part of the export and are not restored.
What format should I save it in?
Whatever your tool exports, as long as the same tool or a compatible one can write it back. A screenshot of the content is better than nothing and worse than an export.
Do I need a backup of a tag I am about to discard?
If the deployment is finished, no. If you are replacing the tag because it is failing, yes, since the replacement needs the same content.
Is reading a tag enough to back it up?
Reading and looking is not the same as reading and saving. The export has to end up somewhere that survives closing the application.