How price per terabyte is worked out

In short: every page is ranked by what a terabyte costs with delivery included, the figure is left blank when eBay quotes no delivery cost, and auctions are ranked on their current bid.

Price per terabyte includes delivery

One number decides the order of every page here: what a terabyte costs. It is the delivered cost divided by the total capacity on offer, in decimal terabytes. That is the unit drives are sold in, where 1 TB is 1,000 GB rather than 1,024.

A listing of five 2 TB drives is ten terabytes, so its price per terabyte is the price of the whole lot divided by ten. The capacity column shows both the total and the drives that make it up, so a lot always says what it is a lot of.

Drive counts work the same way: a lot of five counts as five drives. A listing that only says how many the seller has in stock, such as "qty 10 available", is priced per drive and counts as one. When a lot mixes sizes, every drive is counted at the smallest size listed, so its price per terabyte can come out higher than the real figure, never lower.

Blank rather than wrong

When eBay quotes no delivery cost, usually because it is calculated against a buyer address that HardDiskIndex does not send, the price per terabyte is left blank rather than worked out from the item price alone.

A confident number built on a missing one would sort to the top of the cheapest page, which is the one place a wrong answer is most visible and least questioned.

Auctions are ranked on the current bid

Auctions are included and ranked on their current bid, marked at current bid in the Per TB column. An opening bid works out to a very low price per terabyte, but it is not a price anyone can pay, so the label sits on the column being sorted rather than tucked away beside the price.

Every listings page has a control that hides bid-only auctions.

Accessories are filtered out

The eBay category that holds drives also holds caddies, trays, enclosures, cables, labels and carrying cases, and many of them quote the capacity of the drive they fit. They are filtered out: an empty case advertising "1TB-6TB" is not six terabytes of storage.

The filter is re-applied to the whole catalogue each time listings are collected, so a case that slipped through under an older rule is removed rather than left in place.

Specifications come from the title

Interface, form factor, rotational speed, recording technology and part number are read from the listing title, because eBay's search API does not return item specifics. Where a title does not say, the column is blank rather than guessed.

Recording technology is shown only when the listing states it. No vendor publishes a reliable list of which models use which recording method, and shingled recording (SMR) is the specification buyers have most often been misled about. Showing a blank as "conventional" would mean HardDiskIndex asserting something it does not know.