Mac OS Sierra - Can't merge former time machine partition on external hard drive I have an external hard drive with two partitions. One partition was used to save files and the other was dedicated to Time Machine. Now I have a new hard drive which I use exclusively for Time Machine, so in the first hard drive I simply want to delete the Time Machine partition and merge its empty space in the other partition.

The problem is that using Disk Utility I was able to delete the content in the Time Machine partition, but I can't merge it. The minus sign which I would normally use to remove a partition is greyed out and there's a message saying "You can not remove the first volume on the disk".

I read that using diskutil from the command line might work, but I'm not sure how to do it. 
This is the diskutil output:

~ ❯❯❯ diskutil list disk2
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS Time machine backup     750.0 GB   disk2s2
   3:                  Apple_HFS Files                   1.2 TB     disk2s3
Try First Aid button on  Disk Utility. 
See what will came up. Maybe this way the app recovers / fixes all of your HHD issues. 
Then try to erase (once more ) and merge your disk. 
In many cases First Aid is a necessary step in order to get your job done. 
Cheers.