Does Recovery HD gets affected

Your data in a partition that you hide using Stellar Partition Manager is safe as no operation takes place on the partition's data. In addition, a partition is hidden to avoid access or tampering of data stored in that partition. When you hide a partition, it is temporarily un-mounted.

Note: Only hidden partition can be visible in other applications or utilities but cannot be selected for any read-write operations.

Stellar Partition Manager is Mac partition management software. You can create a new partition, resize an existing partition, delete a partition, or hide or reveal partition. You cannot recover data from any lost/damaged/deleted partition using Stellar Partition Manager software.
Yes, you can split a 100 GB Mac partition (having 20 GB data), into two 50 GB partitions using Stellar Partition Manager. You can resize the existing partition by decreasing its size from 100 GB to 50 GB and then create a new partition of 50 GB from the free space of the drive.

Note: No data takes place when you resize (increase or decrease) your partition. However, Stellar recommends that you backup your data before resizing any partition.

The recovery partition is a feature of Mac operating system that allows you to restore system software of your Mac with just a few clicks, without the use of DVDs or USB drives. It is a small (approx. 650MB) partition on your Mac’s internal hard drive that is ‘hidden’ and reserved for common utilities such as Disk Utility. Stellar Partition Manager does not perform any operation on the Mac recovery partition and hence does not affect it in any way.
General preferences allow you to set tasks that occur at the end of any process in Stellar Partition Manager software.

To change finish task preferences:

Click General tab in Preferences screen.

In After finishing task section

1. To assign a sound that the application plays at the end of process, select Play Sound option. Click Browse to browse to a saved sound file and select it.

2.To send the system to sleep at the end of process, select Send system to sleep option.

3. To shutdown the system at end of process, select Shutdown system.

4.Click OK to save the settings.

To change update task preferences:

1. Click General tab in Preferences screen.

In Updates section,

1.To assign a frequency for checking of updates, select Check for updates option.

2.Select frequency in the list.

Click OK to save the settings. To check for current updates, click Check for updates now.

Monitoring preferences allows you to choose fast shifting of a volume by Stellar Partition Manager with or without verification of the moved data written at a new location. Verification of data helps in preventing data loss. Fast shifting performs the task quickly.

To verify the write process after completion of task:

Select Verify After Writes option.

Click OK.

Note: Verify After Writes option will slow the process of fast shifting.

For fast shifting of the volume:

Select Fast shifting option.

Click OK

It is possible that your Mac partition has bad sectors in it. You must first check the partition for bad sectors. Additionally, if the partition has some errors in it you may not be able to re-size the partition.