A friend from London told me that the receipt might not be necessary for me to avail of the warranty given by such brand company. All I have to do is make sure that my product is registered online. If the time comes that I need a disk replacement, then I just have to contact the company, send them the disk and comply with other admin procedures. I'm not sure of the accuracy of what he said but according to him, he has personal experience of asking for a disk replacement and no receipt or other purchase document was asked from him.
There wouldn't be much issue as to ownership anyway since the serial number of the item is needed when registering the product online. (Wait, unless my serial number is indicated on the receipt and someone got hold of it, and someone who is crazy enough to register it in his/her name? Ooookay.) So I just have to make sure that no one else gets the serial number? I didn't realize the importance of serial numbers up until now.
But still, there is no absolute certainty that absent the official purchase receipt, I will be able to avail of the disk replacement warranty that came with the product. And more so when the product registration online asks for the specific date that I bought the HD which, of course, escapes me.