How to merge two series-separated partitions in one hard disk.

[Yuyang Wu]

Hello

I have an external 500G disk and there are two partitions inside currently, first one is APFS for storage document and second one is macOS(HFS+) for time machine backup. Now I'd like to move some space from the backup to document partition, however, I couldn't merge the third partition that was divided from backup partition to the document partition by build-in disk utility app. My macOS version is 10.15.4. The third partition just merge back to the backup one if i click the 'minus' button. What should I do now?
(i am a newbie into English tech range if there's any inappropriate vocabulary please point out them I am appreciate!)

 

[Taz Mangus]

Because the 2 partitions are not the same format (one being HFS+ and the other being APS) it isn’t possible for it to be merged as it is.

 

[Yuyang Wu]

Because the 2 partitions are not the same format (one being HFS+ and the other being APS) it isn’t possible for it to be merged as it is.

I reformat it to Apfs but it doesn't work

 

[maxmdean]

I reformat it to Apfs but it doesn't work

Did you ever find a solution for this? I am having the same problem.