Using disk utility to expand an already existing partition MAC OS 10.15 I have an external Hard Drive that has around 5 partitions. Among them there is a partition, say, Partition 1 of 500GB. There is another partition, say, Partition 2 of 20GB. I want to get 100GB of Partition 1 and add it to Partition 2. How can I do that? Both the partitions contain data, (partition 1 has around 100GB of data only), so I can't erase them and make them again. Is there any way to do it?
The answer is no. This answer assumes the partitions are in chronological order. This is one of the problems which was solved by APFS. With APFS, more than one APFS volume can reside in a partition and the volumes can share the space.
In your case, you would need to shrink the a partition by 100 GB. There a tools provided by Apple or third parties that can shrink the volume(s) stored in a partition. The partition can then be set to a smaller size, thus creating free space below the partition. However, adding the free space to the next partition usually requires first moving the partition up by 100 GB. This can not be done by using the Disk Utility. Once the partition is relocated, then depending on the volume format either the Disk Utility or a third party tool may be able to enlarge the partition and containing volumes(s) to contain the free space.
Before attempting to move a partition, a backup of the partition should be made. However, once a backup is made, generally moving the partition becomes unnecessary. An alternative would be to delete the partition and created a new partition of the required size, then reloading the partition from the backup.