Two Part Numbers on One RFQ: Stopping the Substitution

Two part numbers on one request for quotation read as one instruction to the buyer and as one permission to the seller. That gap opens before anyone discusses price, usually inside a single closing sentence asking for the best price on the whole lot.

The pair used here is Samsung's PM893, a SATA 2.5-inch data center SSD, and PM1653, a SAS 2.5-inch enterprise SSD. They are named only to give the two lines something concrete; any pair differing in interface, generation or endurance class behaves the same way.

The list, in the shape it usually arrives

A mixed RFQ is not a mistake; a mixed RFQ compressed into one undifferentiated line is. Consolidating the envelope saves everyone a week of email. Consolidating the line saves the seller an argument later.

What lands in an inbox most weeks:

• 60 × PM893 — batch A, SATA-only backplane
• 40 × PM1653 — batch B, SAS backplane
• 100 drives in total, one shipment

"Please quote the best price for the whole lot."

Read it the way a seller reads it: two numbers, one quantity, one price, one delivery. The two notes are the only part of the message that makes the mix binding, and the closing sentence tells the seller that the mix is precisely what is not binding. A seller who wants the order will find the cheapest way to deliver a hundred drives, because a hundred drives is what the message asked for. Mixing families across a plan is legitimate and is covered in building a realistic sourcing plan from mixed brands, models and capacities. The error is not the mix. It is the missing boundary around it.

Where the two numbers are physically different

The differences that survive negotiation are the ones built into the connector, and they are the ones a substitute cannot walk across. Endurance and market position matter too, but only once the drive is in a bay that took it.

The asymmetry underneath the table is worth memorising: a SAS backplane will take a SATA drive, a SATA-only backplane will not take a SAS drive. That one-way door decides what a substitution costs. Deliver SAS where SATA was ordered and the carton is scrap in your own warehouse, discovered at installation. Deliver SATA where SAS was ordered and the machine boots, passes the smoke test, and quietly runs on one path where the design assumed two.

The dangerous substitution is the one that works.

When a substitute is a substitute, and when it is a deviation

A substitute is only a substitute if you agreed to it before it shipped; otherwise it is a deviation with a tracking number. The distinction decides who pays for the fix.

1. The bay cannot take the unit without an adapter. Reject the line and ask for the ordered number. An adapter adds a failure mode that neither the chassis vendor nor the drive vendor owns.
2. The bay takes it, but the design loses something it needed. Re-quote instead of accepting a discount. A discount computed against the wrong drive is a different offer, and whoever sized the multipath finds it first.
3. The bay takes it, the design does not care, and the unit is genuinely interchangeable. Accept only in writing, per line and per quantity, at the substitute's own code and price. Silence here sets the default for every future order from the same desk.

One case deserves an explicit yes: the higher-capability number going into a chassis that can use it, priced on its own line, approved before dispatch. What is not acceptable is the same swap arriving as a fait accompli on an invoice line reading "or equivalent".

Why a blended total is not a comparison

A blended unit price describes the mix you were quoted, not the goods you need, and the mix is the one variable a seller can change without touching a single number on the price list.

Two sellers quote the same hundred drives. One ships 60 of the cheaper number and 40 of the dearer; the other ships 80 and 20. The second blended price is lower, and not one of their prices is better. They have not become cheaper at anything. They have moved twenty units of your order onto the line with the smaller number beside it. Comparing the blends compares two different orders and calls the difference a price.

Weighting by unit is wrong even when the mixes happen to match, because it still assumes any unit can stand in for any other. The weight that carries information is the count of slots that accept only one of the numbers — the bays, nodes or trays that will not take the other. Cost per correctly filled slot is a comparison. A unit in the wrong slot weighs zero, not one: that slot is still empty and you now also own a drive you did not order. The whole reason the RFQ carries two lines is that the units are not exchangeable.

If a seller refuses line prices, ask for two totals — the total as written, and the total with the mix reversed. If the second is lower, the intended substitution is visible, and you found it before anything was packed.

Writing the line so it cannot be swapped

One part number per line, one constraint per line, one price per line — that is the rule, and the fields below exist only to enforce it.

1. The full order code, never the family name. PM893 and PM1653 name families; the code on the label is the purchase, and it belongs in the quotation complete, beside the quantity.
2. The constraint, in your own words. "SATA-only backplane, eight bays per node" does more work than any adjective, because it gives receiving a rule to apply when a carton looks wrong.
3. One named alternate, or the word none. A field reading "equivalent acceptable" reads "seller's choice". If a second source is wanted, name it with its own code on its own line.
4. Quantity per line, plus one sentence: "No substitution, partial or complete, without written approval before shipment." That sentence has to survive negotiation intact.
5. Line price and line total. A quotation showing only the blended figure cannot be audited at the dock or compared beforehand.
6. Lot expectation per line. If both lines feed one spare pool, state whether each must arrive as a single lot; the reasoning is in why batch consistency matters for server SSDs, RAM and HDDs.

An unglamorous question hides under all of it: which chassis in your own rack will reject which number. That is a fact about your hardware, not the seller's catalogue, and it is cheaper to settle before the RFQ leaves your desk. Send the chassis list to us and we will work through it bay by bay. The same request separates a real promise from a hopeful one, which is the subject of what a quote for 100 drives actually commits to.

Checking at the dock

A carton count that matches the total but not the lines is the most common way a substitution survives all the way to installation.

• Count by line before you count the total. When the total is right and a line is short, stop the receipt. Installing first turns a shipping dispute into a used-goods dispute.
• Read the label against your line, not the packing list. The packing list was generated from what the seller shipped; your line is the only document generated from what you asked for.
• Look at the connector once per line. SAS drives carry a visible bridge between the data and power sections; SATA drives do not. No power, no tools.
• Power one unit from each line and read what it says about itself — interface, capacity, firmware revision, power-on hours. Two lines reporting the same interface are either the same drive or one is mislabelled, and both need a decision before the pallet is unpacked.

Photograph each line's carton label and one unit from each line at unboxing. A photograph taken before the seal breaks is evidence; the same photograph taken a week later is a description.

The claim underneath all of it is modest. A mixed order is not a risky order. A mixed order written as a single line is.