Upgrade HDD or get new device

Well, if you get a new device, such as an NV+ v2, Ultra 4 or Ultra 4 Plus, you would still need at least the 1TB drives you mentioned. In that case the bill would be around $800 at least ($395 for the NV+ v2 and $408 for 4xST31000524AS) based upon current Netgear prices.

I personally would recommend that you work toward an Ultra 4 (x86 based units have more flexibility and options, but costs around $145 more) and keep the NV+ in service as a backup device. You can schedule daily automatic backups via rsync that only take minutes. (Initial backup needs to be done via NFS and will take hours). The rsync process synchronizes the files between two units and only the changed sectors are updated, as opposed to the entire changed file in incremental backups.

I finally got to this point two years ago, and would not want to go back. Because the backup target is smaller, certain shares with non critical data (mp3 files that are duplicated elsewhere for example) would not need to be backed up as opposed to personal financial data and photos.