External hard drive is mounted as read only

It sounds like the destination you have chosen is read-only. As @Barney-15E mentions you can only store items within your home user folder or within the "/Users/Shared" folder. Any area outside of the "/Users" folder is a read-only system area that cannot and should not be modified. See these two Apple articles for details:

If you are having issues transferring the items to the internal drive within your home user, then are you using the Finder to transfer the files or some other app?

As for First Aid not being able to repair the external volume, click "Show Details" in First Aid and scroll back through the report to see if any details of the error are listed. Since the 8TB drive is using the HFS+ file system, you could try using the paid third party app Disk Warrior to attempt repairs. Disk Warrior is usually able to fix most HFS+ issues as long as the hard drive(s) are healthy. Keep in mind doing any repairs is risky if you don't have a backup of the data on the drive being repaired.

To check the health of the hard drives you can use DriveDx. Post the reports for both drives in the RAID here using the "Additional Text" icon which looks like a piece of paper. For an external drive you will need to install a special USB driver. However, even with the special USB driver some USB chipsets used by external drives may not allow the necessary communication to access the drive's health report.

As far as the EtreCheck report, I see you have CleanMyMac installed. Anti-virus apps, cleaning/optimizer apps, and third party security software are not needed on a Mac. These types of apps usually cause more problems than they solve plus they impact system performance. CMM is one of the most notorious apps causing problems with macOS. macOS has great built-in security especially when users practice computing habits as described in this excellent article written by a respected forum contributor:

Effective defenses against malware and ot… - Apple Community

I'll try to repost the EtreCheck report for you to see if others can read it:

You say the drive is setup in a RAID 1 configuration.

Where is the partner drive?

It could be that the RAID is degraded since the second drive in the mirror is missing.

What file system is on the external drive?

Is the external drive being used for anything else except a basic data drive?

Make sure you have "Ignore Ownership" checked on the permissions of the external drive. In the Finder, use "Get Info" on the drive where you should see this option.

Has anything recently changed with your OS, software, or hardware?

3Design wrote:

I cannot reformat this drive as it contains sensitive files that are too large to be moved onto a separate device. Is there any way to find out what happened?

I hope you do have a backup of the important data located on this drive. If not, what happens if the drive fails, or the data on the drive becomes corrupted or inaccessible, or something is accidentally deleted?

You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.

Would EtreCheck help to give you a better overview?

Possibly.

Where on your internal drive did you try to move the Photos Library? You essentially only write to your home folder. Most other places cannot be modified.

What is the file system format of your external drive? It will show that in Get Info.

Is there anyway I can forcefully change this to Read & Write?

Seems like there is a problem with the drive, so I don't know what would be possible.

Many here have suggested DriveDX to fix drives that Disk Utility cannot.

Have you tried copying the data again since the first time. You stated you got a Read Only error when you tried to copy to your internal drive, but that would normally only occur if you tried to copy it over to a system area on the drive, like the top level. I would try to copy the data to somewhere else before I tried repairing it.