It's worth reviewing minidisc's shortcomings and ultimate failure to achieve its potential as a consumer format, if only so future engineers of future technologies have an idea of what to do, and what to avoid.
the roots of minidisc's failure
Sony's mistake as the inventor of minidisc was to keep the format proprietary.
Manufacturers such as Sharp had to license technology to produce competing players. I'm not a free market evangelist but this strategy clearly hampered the medium's spread. This tactic also presumably led to the unfortunate lack today of anyone other than Sony ever offering HiMD portable recorders (or, in fact, much HiMD gear at all).
Worse (and exacerbated by the lack of true competition) is that Sony was very slow to unlock the medium's actual potential. In another brighter universe minidisc became a universal replacement for both the cassette and floppy disk around 1999. It didn't in ours because Sony would not allow convenient transfer of data on and off of minidisc, out of fear of music piracy in the absence of evolved digital rights management (DRM).
There's a sordid saga lurking here, namely Sony's long internal conflict between engineers and lawyers. But regardless of that battle's causes, its main consequence has been a minidisc format that caused users unnecessary headaches. Sony's resistance to the unimpeded flow of data on and off minidisc affects users negatively to this day, even as the format is retired.
the problem of moving minidisc recordings to a computer
Minidisc portables suffered from day one because they did not provide either a digital output (of any kind) or a data port (such as USB or Firewire) through which it was possible to transfer recordings to a computer.
There were work-arounds for this limitation but none was appealing. Option zero was to simply transfer recordings in real time via a portable's analog output: to play the disk and re-record it on a computer. This yields acceptable results but introduces generation loss.
Option one was to invest in component-stereo-sized minidisc deck (such as my own Sony MDS-JB920) which had both coaxial and optical digital outputs, and transfer recordings digitally again, in real time. This is still my strategy of choice actually. It provides lossless transfer of recordings, at the expense of a second minidisc player (and a soundcard with digital inputs) and a lot of time.
A third little-known option was to invest in a proprietary Minidisc Transfer Editor (MDTE) system by the British company EDL, which allowed faster-than-realtime digital upload of recordings but required special hardware (a repurposed an ancient SCSI Sony MDH-10 with custom firmware). This system had other advantages (such as the ability to recover data from mindisc with corrupted TOCs, see below) but was never intended for the average consumer and I've determined that its software decoding of ATRAC is inferior to that provided by my Sony hardware.
(For exhausting detail on transfering recordings, see my ancient wisdom below.)
HiMD made transferring recordings easier
HiMD portables offered several steps forward. Sony did, little by little, open her kimono and permit increasing levels of data accessibility as HiMD users agitated in light of the freedoms afforded by mp3 players and the like.
First- and second-generation HiMD portables originally permitted limited uploading of one's own recordings via USB, but with constraints; over a few years unpopular limitations (such as the inability to upload a given recording more than once(!), or upload recordings made from digital inputs) were loosened.
Sony's third-generation HIMD recorders removed most of the remaining constraints and at long last allowed uploading of old (non-HiMD) minidisc recordings in addition to unlimited uploading of new recordings, all over an improved higher-speed USB connection fulfilling a fantasy of some minidisc enthusiasts that dated back almost ten years.
(The only unresolved limitations are on little-used high-compression 'NetMD' modes, I believe.)
...but introduced an insidious, catastrophic Achilles' Heel
Despite these advances, however, even last generation HiMD recorders retain a crucial weakness: recordings on the disks themselves are encrypted.
While Sony permits recordings on them to be uploaded and converted into an open file format (.WAV), doing so requires Sony's software, which is (or was?) available only for Windows computers; Mac users continued to suffer a more limited feature set provided by different software.
(Sony even sold physically-identical HiMD recorders under two names to users of Windows and Mac machines, the only difference being firmware tweaks that allowed one to communicate with Macs!).
(Users of other operating systems like Linux are simply SOL as far as I know with HiMD.)
Unless Sony ever decides to open their DRM/encryption scheme, HiMD files are not completely trustworthy, as long as they remain archived only on disk. In other words, one of the long-standing absolute advantages of the medium, that it was a very reliable archival medium, was nullfiied by DRM games.
why the encryption on HiMD is unforgivable
The downside to random-access non-linear technologies and this goes for memory cards and HD as well as minidisc is that they require a TOC (Table of Contents) somewhere on the medium to keep track of where your sound is.
The reason a TOC is a downside is that if anything happens to that table, your data is not reachable. It's still on your disk, but your player won't play it and your software can't transfer it. (This is not uncommon problem for computer users, who know the TOC as the File Allocation Table (FAT)...)
TOC loss has in fact been a non-theoretical issue for me: when the contacts of one of my Sharp's external battery case became worn, I lost several recordings because the unit lost power before completing TOC update.
With the first generation of minidisc TOC loss was at least a somewhat solvable problem, because recordings were not encrypted. With some clever hacks, data could often be recovered; minidisc.org is a great resource for reading about how (the most common technique being 'TOC cloning').
Users such as myself of EDL's proprietary MDTE system could also scan disks for valid audio data and reconstruct a TOC (or at least, copy data off the disk); this is a service the company offers commercially in fact.
The situation is significantly more dire with HiMD. Because the sound data on HiMD is encrypted, only Sony has the (theoretical) ability to reconstruct a TOC, or even recognize and extract valid audio data, from a HiMD disk once its TOC is compromised.
And Sony's reaction to requests for assistance to do precisely this have been completely uninspiring. As far as I know Sony essentially washed their hands in this regard, I have heard the accounts of several recordists who have lost their recordings, apparently for good, this way. I find this completely unacceptable.
Fortunately, be assured that in normal circumstances TOC corruption should not be an issue. If it were, HiMD products would not be in their third generation. But TOC corruption has happened on more than one occasion to HiMD users in forums I frequent. (If, God forbid, this should happen to you, leave your disk untouched and do not reuse it; you may in the future be able to recover it... who knows!)
The bottom line is that if you will be making recordings that simply cannot be lost, you need to use another medium.
other limiting factors
It would have been nice, in the final generation of HiMD, to simply 'mount' my minidisc recorders as removable media over standard USB or Firewire connectors, and to drag-and-drop my recordings as files in universally understood formats, without any proprietary software.
In the absence of such an ability, minidisc remains a non-trivial format to make backup copies of, in all its incarnations.
It is a shame that HiMD was limited to recording at 44.1 KHz and 16-bit samples. As I write elsewhere, there is a real advantage to using higher bit depths (and to a lesser but real extent, sample rates), even though they burn media space faster.
A recurring gripe about all small portable recorders, leveled frequently in its day at mindisc, is that the small buttons common on miniature decks are hard to use, and the small labels and displays hard to read. I don't consider myself particularly ham-fisted but I've found the small buttons on my otherwise much-loved Sony MZ-R37 annoying at times.