need help: how to rescue files on my external hard drive after using Time Machine on macOS

[kangyu]

I just lose my partitions on the external disk after reseting one partition for Time Machine on macOS.

I have an external hard drive with 1 TB, and I made it two partitions:

- one is for some data (named `Resources`);
- one is for Time Machine (named `TimeMachine`).

The partitions are HFS+ format, and I use this setup for many years.

Today, the partition for Time Machine can't be found by Time Machine with my latest macOS Big Sur. It doesn't show in the list of `Select Disk` in Time Machine app. So I decide to "reformat" the partition `TimeMachine` in Disk Utility with HFS+ format again. After "erasing" the partition, it can be found by Time Machine again, and I still keep those files on the other partition `Resources`.

Then the terrible things happened. The Time Machine make a notification said "found a disk for Time Machine"; I follow the hint to enter a password to setup for Time Machine, then the whole disk is reformatted into APFS format, and It has no any partitions. I lose all partitions and files in the partition `Resources`.

I just want to setup one partition for Time Machine, but it erases and reformats all disk without asking me.

Now I just want my files back. I know my files are not missing. It just loses the index table for finding them on my disk.

Is that possible to use `testdisk` to rescue my files in this scenario? If true, how to get started?