I'd consider taking a wider look at your backup strategy - you'll want to update it over time and you'll also have other data you want to look after too. You may find that 500GB is fine for now, but you may need 2TB in a few years.
For a local backup, I'd go with
@floppybootstomp's suggestion and get yourself an external drive or RAID dock/NAS and backup the data to that. The data doesn't get corrupted due to inactivity, so that's not a worry. If you're accessing the data very infrequently, perhaps get two USB external drives and back up to each one - it'll save money and complexity compared to a RAID array.
The alternative is to use a cloud based service like
@muckshifter suggested - you can get a free 1TB storage account via Dropbox.com, and they'll handle storing your data securely. They do this as they want you to subscribe and buy more data, but you don't need to. This may not be suitable if you'd rather not entrust data to someone else, but perhaps it would be a useful complementary strategy.
Using M-disc's is great if you need to write a one-off archive that needs to last for a long, long time - but surely your photos are going to be a constantly updated collection? A mixture of online storage and local backup would be my choice
. If I had to make a huge archive catalogue then M-discs are a good way of ensuring stable longevity, but for your photos I think there will be better choices.