Why did time machine ask me to erase my disk when the seagate external drive is for both mac and windows?

Hi rishma225,

Your external drive most likely came formatted with the ExFAT filesystem. That format is indeed compatible with both Macs and Windows PCs, as both can read and write with that format.

That said, Time Machine requires additional filesystem features that ExFAT doesn't support. Because of this, it erased your external drive and reformatted it to use a filesystem that is designed for macOS (one of the two below):

Apple File System (APFS): If your Mac is running macOS Big Sur (11.0.1) or later, Time Machine uses this format by default. One of the most critical features is the ability to create snapshots, which are read-only copies of the drive at a certain point in time. Time Machine uses snapshots to hold individual backups of your Mac.

Mac OS Extended Journaled (Journaled HFS+): If your Mac is running macOS Catalina (10.15) or earlier, Time Machine only uses this format. One of the most critical features is the ability to create directory hard links - something that almost no filesystem supports, as it can be used dangerously. For safety, only Time Machine can create directory hard links. They are used to stitch backups together, and to prevent copying unchanged files.

Out of the two formats, APFS is the more robust and safer format to use for Time Machine.

The disk needs to be formatted to work properly with Time Machine.