Mini-Mac Server as an NAS

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I would start with a new external array rather than add to the internals. 2TB external drives are now less than $100. I would get a couple of those for data storage and a few more for backup. A program like Super Duper can manage the backups.

I have three 1.5TB drives in a concatenated array so it looks to the OS like a single 4.5 TB drive. The advantage is it is easier to manage the library but if any drive fails you lose everything on all discs. Of course you will have a backup or 2 in case that happens.

Supposedly you can add drives to a concatenated array but the one time I tried it it didn't work properly. The drive got added but it didn't add the extra capacity. I could go to disk utilities and see that the array had 3 drives but I had three 1.5 TB drives in an array with only 3 TB of space. You can't remove a drive so I ended up having to reformat those drives as a new array and restore from backup.

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No, do not make a RAID with mixed internal and external drives. It won't work. Software RAID requires all drives in the RAID set to start up within a short period of time which is very difficult to do with external drives. As soon as one external drive didn't power up in time, that drive would be considered damaged and the RAID set would be in degraded state. I have tried it before. It is very massy. Don't do it.

4TB: Wow, you got quite a music collection! Let us know how long it takes to rip that much music.