A very clever question I believe we are moving past this terminology rather quickly, as it is basically coming down to semantics. Some people may coin any Serial ATA (SATA) hard disk to be conventional, including myself, with IDE now pretty safe to call a legacy-style device. However, this is clearly an error in categorization and labeling, and perhaps even a misnomer, because we are confusing computer bus interface with disk-based data storage mechanics and type of memory. In many cases, we are no longer talking about physical disks! We are describing the interface that the drive uses, and not the mechanics of the drive. For instance, a solid-state hard drive has no moving parts, but still uses, at least in most cases, Serial ATA (Serial Advanced Technology Attachment). Likewise, while IDE drives could be coined "legacy" devices due to the rise of their more reliable SATA replacements, we are again falling into a labeling problem and an issue with semantics and terminology. Solid state drives can be found utilizing serial ATA, USB, serial attached SCSI (SAS), parallel SCSI, PATA IDE, and so many more computer bus types, we may not be able to list them all. NAS (network attached storage) as a file-driven network medium confuses the situation even more. While not a computer bus, it behaves very similarly on a macro-level.
How do we develop terminology for devices? The proper terminology can go way over peoples heads, and this is why we often revert back to terminology everyone sort of understands: conventional SATA, IDE, SSD. We tend to label hardware into groups and ways in which we understand them, but there are physical properties that govern the mechanics of these devices. I believe that labeling hard disk drives with moving platters that use the SATA bus type as "a conventional SATA hard drive" can be considered OK and appropriate when giving a presentation meant for a wide audience. Most people understand that, in the context of the discussion or presentation, that "conventional" introduces the idea of a hard drive with non-volatile storage, random access, and platters. "SATA, SATA2, or SATA3" signifies to the average computer user how much throughput the drive is capable of, how fast it performs, and how well it is conditioned, without the necessity of completely understanding certain key aspects such as read/write/seek, random access times, memory cache, RPM, and other variables that could be even more determinate in how well a drive functions. For instance, drives operating at less than 7200 RPM are still being sold with laptops. Likewise, hard drives are still being included on some desktops that lack a large cache or under perform in access times.
As time goes on, and as the line between volatile and non-volatile memory blurs, we know why this semantic phrasing will not work. The term "hard drive", itself, has come to have multiple meanings and originated as a way of distinguishing the storage technology from systems that relied on external mechanisms just to boot. Thus, younger computer users, as well as less experienced ones, should be aware that there was a time when computer systems had no internal data storage functionality at all. Of course, today, even the term "internal" has become subjective! We could go on.