
Windows uses NTFS file formats, while Macs, up until the introduction of Sierra, have used HFS+. In the Terminal window, type df –h and hit Return. The recent Western Digital "easy store" and "My Book" hard drives have a Mac Sierra 10.

Format External Hard Drive to Supported Format. You’ll have to reformat your USB to a file system Mac does recognize.

It seems IO controllers on the M1 don't work very reliably. Internal errors – file system error, outdated drivers, unknown errors, etc. If you completed a previous backup successfully, verify that backup: Press and hold the Option key while choosing Verify Backups from the Time Machine menu in the menu bar.

I recently bought a XPG SSD (SX6000 Pro 1Tb) and an Orico case, to use as an external SSD drive.If your external hard drive shows the wrong capacity, you will see less space than actual.M1 mac not recognizing external hard drive.
