So when you are planning to drop any disk from the diskgroup, we need to see if the data in the disk we are removing can be readjusted in the other available disks in the same diskgroup.
When you drop a disk from diskgroup ASM will seamlessly migrate the data to the existing disks in the disk group.
From the above output, diskgroup 4 has three disks each with the same size. If you see the FREE_MB, it is almost same in all the 3 disks.
Now we will drop DISK_F and DISK_G from diskgroup 4 and will see how the data is rebalanced in the last remaining disk DISK_E.
Now we can see from the above result that the FREE_MB for disks DISK_F and DISK_G is increasing as data is moving to DISK_E and FREE_MB is decreasing in DISK_E as data is coming from other two disks to this disk.
When the total rebalncing is completed,the disks will be removed from the diskgroup 4.
Hope it helps