PC storage standards: “Hard cards” (early HDD-on-card solutions) interfaced to the system using which expansion bus specification?

Introduction / Context:
Before ubiquitous 3.5-inch drives and SATA, some manufacturers integrated a hard disk and controller onto a single expansion card called a “hard card.” Understanding which system bus these cards used is a matter of PC hardware history.

Given Data / Assumptions:

• Desktop IBM-PC/AT era architecture.
• Common internal buses: ISA (8/16-bit), later PCI (not used for early hard cards).
• Goal: Identify the bus spec that hosted hard cards.

Concept / Approach:

Hard cards plugged into the motherboard expansion slots and presented as an integrated controller+drive. The prevalent general-purpose expansion interface at that time was ISA (Industry Standard Architecture), available in 8-bit and 16-bit variants.

Step-by-Step Solution:

Verification / Alternative check:

Technical manuals and ads from the period list “Hardcard” devices as ISA add-ins compatible with AT-class systems, not PCMCIA (laptop) nor SCSI unless using a separate SCSI controller card.

Why Other Options Are Wrong:

• SCSI: A peripheral interface; a SCSI hard card would still plug into an ISA SCSI adapter, but typical hard cards were self-contained ISA devices.
• PCMCIA: Laptop credit-card-size standard, irrelevant to desktop hard cards.
• MFM: A magnetic encoding/drive interface type, not a system bus specification.
• None of the above: Incorrect because ISA is correct.

Common Pitfalls:

Confusing storage encoding/drive interfaces (MFM/RLL/IDE) with the motherboard expansion bus; assuming SCSI due to external devices without considering slot form factor.

Final Answer:

ISA