Where can I find a reliable external HD compatible with a 14 yr. old Mac 10.6.8 with a USB 2.0 port (and adapter)?

The now selling Seagate Backup Plus and the WD "My Passport," for example, require Mac OS X 10.11 and OS X 11, respectively.

That refers to the software that ext drive manufacturers want you to install. Most is garbage and need not be installed. Time Machine and basic transfer functions work fine without the dreckware the manufactirs foist on users.

For whatever it's worth, I read this at PC Magazine: "The only case with hard drives where the USB standard matters much is if you connect a drive to an old-style, low band-width USB 2.0 port. Also, if it's a portable drive, that USB 2.0 port may not supply sufficient power to run the drive."

There is some truth in that. "Name-brand" drive enclosures (not the drives inside them) tend to be designed to go on sale every other weekend rather that give long, reliable service. Also, as electric motors (like the one that spins the drive's platter) age, they can begin to demand more power. As a bus-powered drive must get ALL power from a USB port with power limits, that is a common point of failure.

Also those "portable" drive have low-speed 2.5-inch laptop-class drives that not very robust. The ONLY hard drive failures I've work in over 30 years of using computers with hard drives were with 2.5-inch laptop drives. When I worked in law enforcement, we called that a "clue."

If you must buy a cheap name-brand drive that goes on sale every other weekend:

• Use Disk Utility to erase and reformat the drive as Macintosh HFS+ (for your OS version). Most come with Windows formatting or special proprietary formatting that seems more likely to belch up a low-power problem.
• Buy a POWERED USB hub (comes with its own power supply) and place it between the cheap drive and the computer to take the load off the USB port.

My approach? After suffering grief with cheap bus-powered drives on our Macs and those I support, I finally asked, "How much is my data worth?" and decided to buy a better product. For our desktop Macs I now use only OWC Mercury Elite Pro external drives that contain proper desktop-class 3.5-inch drives. They have a an independent power supply so the low-power issues is eliminated.

This is it: https://eshop.macsales.com/item/OWC/ME3NH7T01/

The iMac compatibility list in that web page goes as far back as Late 2009 iMAcs but I've used that series on Macs as old as a G4 PowerMac from 2003.

That's a pro-class drive and most of the long-serving contributors here recommend them as well. As desktop computer has little need for a "portable" drive like the WD PAssport, go pro. Our oldest OWC exteranl drive is now between 15 and 20 years old and, although now relegated to light-duty due to great age, still works as wella s it did when new.

Disclosure: I have no connection OWC or its subsidiaries other than as a customer.

Some of those requirements might only apply if you intend to use the software that comes with the drive. (E.g., software to do file encryption, or other such stuff.)

If you're just going to use a drive for storage, you don't even need to buy a "Mac" drive. The difference between "PC" hard drives and "Mac" hard drives is generally that the "PC" drives come pre-formatted with NTFS and the "Mac" drives come pre-formatted with HFS+.

i just buy a drive, copy off any pre-loaded files on it in case I need them later (which I almost never do), and then go into Disk Utility and format the drive using HFS+ (Mac OS X Extended (Journaled, NOT case-sensitive)).

If your files don't take up enormous amounts of space, another way to transfer them might be to copy them to a USB flash drive or a memory card, and then copy them from that flash drive or card to the new machine. 32 GB, 64 GB, and even 128 GB cards and drives are common these days, and even 32 GB of space is enough space to store a LOT of Word documents.

Don't rely on flash drives and memory cards for long-term backup, though.

I think you're going to be hard pressed to find one NOT compatible.

You seldom need any 3rd party software unless the drive is purpose built. Macs have all you need and generally they work far better without the unneeded software.

If you want to use an external startup drive on a Mac that lacks USB 3.0, but has FireWire, a drive with dual USB and FireWire interfaces is definitely the way to go. FireWire for performance now, USB 3 for being able to easily connect the drive to a modern Mac later.

When my iMacs' internal HDD filled up to the point that everything became horribly slow, I put a 2.5" SATA SSD into one of OWC's portable 2.5" Firewire/USB enclosures. Firewire 800 bottlenecks a SSD's sequential read & write performance, but it worked out well enough, and saved me from having to do (or pay for) extensive iMac surgery. I believe this is the enclosure:

https://eshop.macsales.com/shop/firewire/on-the-go

A bit more expensive than Allan's suggestion, but along the same lines...

https://eshop.macsales.com/shop/firewire/1394/USB/EliteAL/eSATA_FW800_FW400_USB

Yeah SSDs on FW800 bench about 800 mb/s, USB3 varies more but likely average 1,300 mb/s, the zero Track Seek time with SSDs make even the slow USB2 a decent boot option.