A new SSD usually needs to be formatted first.

macOS:

1. Open Disk Utility (in Applications > Utilities).
2. Under External, select the SSD.
3. Click Erase.
4. Choose a format: APFS for Apple devices, or exFAT for use with both Apple and Windows.

Windows 10/11:

1. Open File Explorer and go to This PC.
2. Right-click the SSD under Devices and Drives and choose Format.
3. Choose NTFS (Windows only) or exFAT.