Coo wrote:does this mean I need to buy 2 HDD's, 1 to back up the other, a backup for a backup?
You have to do your own risk-analysis of exactly which risks you are trying to mitigate, and how much you are willing to pay in money & time, to reduce your chances of being affected by those risks. For example - having data on 2 disks does reduce the chances of data loss due to some risks (e.g. disk failure, although multiple disks can still fail within a short space of time, and you have to consider how you will detect deterioration of the disks) - but if the disks are located in the same house, then fire / flood / theft etc. may still affect both copies.
Also, if you have a hardware fault that causes silent data corruption, then you might propogate that corruption onto your backup copy. This is where having multiple generations of backups would need to be considered - at the cost of time and money, as I mentioned, if this is a risk that you want to consider & mitigate.
Online backups have risks too - service providers could disappear, or lose your data, or you could lose your online connection when you need access to your data. And is your online connection upload speed suitable for the amount of data that you want to save to an online backup? These are just a few of the things to consider.
One thing you said sounds like this disk was not a true backup, however:
Coo wrote:so much for a stress free backup system for in case my PC dies!
If this disk really was just a backup of your PC, and your PC is still OK, then you wouldn't care about this disk, since the data is still on your PC.
Perhaps you mean that some data exists
only on this disk, after it has been deleted from the PC i.e. some data on this does no longer exists elsewhere? In that case, the disk was not a backup - it was a single copy, and therefore a single point of potential data loss.
Good luck and hopefully you'll be able to get your data recovered.