There's an enormous amount of misinformation on this subject. It's like memory effect in NiCd batteries in that some of the described effects are real, but only apply in minutely specific circumstances, and are very rarely the real cause of reported problems.
Disks do not become less reliable when nearly full, or at least not significantly. They do sometimes become slower when nearly full, but again, absent other concerns, not significantly.
The issue is that there almost always are other concerns, mainly fragmentation, the effect that Louis describes. This occurs when files of various sizes are written and deleted as a disk is used. Beyond that, hard disks are mechanical devices and subject to wearing out.
Almost all of the effects in hard disks assumed to be caused by various highly specific and esoteric usage patterns are actually caused by either fragmentation, which is repairable, or wear and tear, which is not. A disk is most likely to be fragmented when it's full or nearly full in most usage scenarios. A disk is more likely to be full when it's old than when it's brand new. In neither case is the performance or reliability problem actually caused by capacity issues.
It is quite correct that many of the alleged disc faults are simply caused by excessive "thrashing" as the read and write heads are continually doing a "hunt-and-peck" job to reassemble all the data into the required contiguous stream for playback, or pigeonholing the data packets into widely scattered blank sectors.
You should always start every day with a freshly formatted drive, mechanical or solid-state, and iny case, simply bulk-copying a fragmented drive onto a freshly formatted one will fix a lot of those problems like magic.
For some reason people often seem to think they should use the original disc for an edit source and keep the backup for er, backup. That might be how it worked with film and videotape, but with digital recording, it's the other way round: Your "backup" is far more likley to be edit-friendly than the "original".
Regarding disk reliability, most of the problems seem to be related to transport and handling. For about 5 years I had a setup with six low-cost Digital Set-top boxes with inexpensive 500GB portable USB hard discs, for doing lossless recordings of Digital TV transmissions, mainly to capture TV commericals for quality checking purposes. Because I never knew precisely when ads were going to be shown, I would simply let the things run for 12 hours or so at a time, and use a simple non-decoding editor to snip out the relevant parts of the Transport Stream.
All I can say is, despite God knows how many hours recording they did, I've never had a single failure, either in the the physical drive or the recordings themselves.
Yet I know of many people (mostly students) whose similar portable drives had a very short and buggy life, from being banged around in a knapsack.