External SSD issue: EFI partition missing after reformatting, now can't be read altogether

[iglaoftheninth]

(MacBook Pro 13" late 2013, 8 GB RAM, i5 2.4 GHz)

Hi all! I recently got a 1 TB Seagate Backup Plus external SSD and was planning to install Mojave on it. After plugging it in for the first time, I opened Disk Utility to change the partition map from MBR to GUID and then started up the installer. The drive's icon was greyed out and the installer told me that I couldn't boot up from it, so I opened Disk Utility again to see what's wrong. I noticed that the EFI partition was now missing for some reason, which I guessed was the cause of the problem.

I wasn't sure what to do at this point, so I tried to erase the drive again, but the process failed with a "couldn't unmount disk" message. Then I tried to erase it via Terminal, but again, the operation timed out while unmounting the drive. What's worse, when I unplugged it and then plugged it back in, the drive didn't show up on Finder or Disk Utility, though it's still listed when I entered the diskutil command on Terminal (only the size, no volumes listed). Any idea what's happening here? Thanks a lot in advance!

 

[DeltaMac]

Try again:
restart your MBPro, with the Seagate external attached.
Go to Disk Utility.
The Seagate should be listed as a device. If not, click the View icon in the top right corner of Disk Utility window, and choose "Show all Devices"
Now, select the Seagate drive (the device), which would be the line that shows the part number or the capacity, and not any line underneath that might show the name of the partition. Erase the drive now. make sure that the erase would make the device format GUID, and that the Format selected is Mac OS Extended (journaled). Choosing the device should remove ALL partitions, and leave you with a single partition, which would then be named with your choice of name.

Don't worry about the APFS, as installing Mojave on that partition will automatically convert your newly-formatted drive to APFS during the install.