HFS+备份数据迁移与恢复相关技术咨询

Hey there, let's break down your questions one by one to clear up the confusion:

1. Did you already lose data by copying to Btrfs? #

Probably not file content loss, but you might have broken the Time Machine backup structure and lost critical HFS+ metadata that Time Machine relies on. Here's why:

• Time Machine uses HFS+ specific features like directory hard links (for snapshots) and extended attributes/resource forks to track backups efficiently.
• When you used rsync without proper flags to preserve HFS+ metadata, it likely copied the raw file contents but missed these special attributes. Btrfs supports some extended attributes, but not all the ones HFS+ uses for Time Machine's snapshot logic.
• The fact that each snapshot folder shows as multi-GB instead of using hard links to save space confirms this: the snapshot structure is no longer intact, so each "snapshot" is a full copy instead of linked files.

That said, your actual user data (documents, photos, etc.) is probably still there in the backup folders—you just can't use Time Machine's restore features properly from the Btrfs copy.

2. What to do with the .HFS+ Private Directory Data directory? #

This is an internal HFS+ system directory that stores the file system's own metadata (like directory records, indexes, and internal structures). When you copied it to Btrfs, it's just a bunch of useless files taking up 70GB—Btrfs doesn't recognize or use this data at all.

• You can safely delete it from your NAS to free up space. It won't affect any of your actual backup data.
• If you ever copy files back to an HFS+ drive, the file system will automatically create a new, valid version of this directory on its own—no need to keep the old one.

3. Can you copy back to HFS+ and use Finder to backup correctly? #

It's not going to restore the original Time Machine snapshot functionality, but here's what you can do:

• If you still have the original Time Machine HFS+ drive, use that directly for any proper backups or restores. Don't rely on the Btrfs copy for Time Machine-specific workflows.
• If you only have the Btrfs copy now, copying it back to an HFS+ drive will get your file contents onto a compatible file system, but the Time Machine snapshot structure is already broken. You won't be able to use it as a functional Time Machine backup anymore.
• Instead of using Finder, use rsync with metadata-preserving flags (-aAX is a good start: -a for archive mode, -A to preserve ACLs, -X for extended attributes) when copying back. This will retain as much critical metadata as possible. After that, you can either manually migrate your user data to a new location or set up a fresh Time Machine backup directly from your Mac.

备注：内容来源于stack exchange，提问作者datafan