How do I identify an adapter cable for a round port on an old hard disk for iMac?

Thanks. In that case, that port is a 6-pin Mini-DIN power connector.

That specific connector is used solely for DC power input, not data. It was Western Digital's way of providing both +12V and +5V to the internal 3.5" drive (typically a WD Caviar Blue or Green), and it requires a matching proprietary WD power adapter.

If you don't have the power adapter that came with it, or it is damaged, its specs are:

• Connector: 6-pin Mini-DIN (same style used in PS/2 keyboards, but wired differently)
• Power Output: +12V at 1.5A / +5V at 2A
• WD Model Number of OEM adapter: Often labeled WDPS036RNN, but a few regional variants exist.

NOTE: If what's on this drive is important to you don’t try to plug in a standard Mini-DIN power supply (like an old ADB or PS/2 cable) — the pinout is proprietary, and incorrect wiring could damage the drive’s controller board.

If your goal is to recover data, one safe alternative is to remove the internal SATA drive and connect it directly to a modern USB-to-SATA adapter or docking station. That’ll bypass the enclosure altogether and usually works without issue. In this case, let me know if you need any further assistance.

This appears to be a 6-pin Mini-DIN connector, commonly associated with S-video, Apple Desktop Bus (ADB), or old PS/2 interfaces, but in the context of an external hard drive enclosure, this is almost certainly a proprietary or semi-standard power or data interface.

Possible uses for this port on an old external HDD enclosure:

• Proprietary Power Connector: Some older external drive enclosures, especially from brands like Iomega, LaCie, or Maxtor, used a Mini-DIN-style connector to supply DC power to the drive. It could carry multiple voltages (e.g., +5V and +12V) required for 3.5” IDE or SATA drives inside.
• Data + Power Combo Port: Less commonly, these ports could also serve as combined power/data connections, especially on older SCSI, parallel, or early USB/FireWire-based drives. This would usually require a proprietary cable that splits into separate power and data lines.
• Serial or SCSI Control Interface: On some professional or niche enclosures, Mini-DIN connectors were used as SCSI ID selectors, serial passthroughs, or even remote power switches.

If you can provide us with the make & model of this device, it would help us identify which of those port types it could be.