Help with Data Migration and re-using drives

I have a 5D3 that has (5) 6TB Drives. Used Space = 16TB 75% and Space = 5.6TB / 25%. Last week one of the drives failed and with 4 drives Used Space was 95%. I replaced the drive and all is good.

I don’t know if this is possible but what I want to do is move data and drives off the Drobo to a Synology NAS as follows:

  1. Copy 6TB of data from the Drobo to the Synology.
  2. Remove a Drive from the Drobo and add it to the Synology
  3. In the Synology add the new 6TB drive to the drive pool
  4. Repeat steps 1-3 2-more times

If I pull a drive from the Drobo it will ask me to replace the drive. If I then remove enough data off the Drobo so it does not need the drive that was pulled will it go green or will it still ask me to replace the drive?

No it won’t be possible. Assuming the NAS will not be JBOD, RAID setups needs 2 or more HDDs from the start, so your Synology unit will need 2 HDDs if not the whole array.
The only way to migrate is to copy off your data, remove the HDDs from your Drobo to your new NAS, format the RAID, and then copy in your data. At least then you will also have an external backup if the unit fails.