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After changing to a new external USB hdd for Time Machine my previous external USB drive used for Time Machine (almost) disappeared. My old drive is a Western Digital, My Passport 25E3 for Mac (1-tb). It is not recognized in:

  • Disk Utility
  • (About This Mac) --> (Storage)
  • **/Volumes$**ls -l

However, I can see the old Time Machine drive here: (About This Mac) --> (System Report) --> (USB) --> (USB 3.0 Bus) -> My Passport 25E3

What can I do to reformat that old Time Machine drive and mount it as just a normal external drive? I've rebooted several times and used different cables.

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I got it.

The problem was that disk had an EFI System Partition. All the work to make the disk readable for MacOS El Capitan was done on Windows 10.
C:\Temp> diskpart
DISKPART> select disk 1
---> clean
DISKPART> exit

note:
(I just noticed that Mac has /usr/sbin/diskutil that I'm sure would have allowed me to deal with the EFI Partition.)

So, I used the Windows Disk Utility to create a 1-tb xFAT partition. MacOS recognized that. Then, I reformatted that to MacOS file format, 1-tb partition. perfect.

As can be seen in the question, MacOS only recognized that disk in 1 place, in the USB device tree. Only Windows seems to have allowed me to see that EFI System Partition and then allowed me to manipulate the disk.

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