Struggling to record and also play HD channels simultaneously

More likely, to the under-the-hood performance of the HDD. The disk firmware can be recovering bad reads or failing writes silently at the cost of increased delay. If that happens in a record or playback operation, the Humax settop program might aggressively time out the operation at the cost of an apparent glitch using low-level AV disk options. A CF add-on program running in the background would just block until the operation succeeds or fails irretrievably, possibly affecting the settop program's AV pipeline.

In such a case, a replacement disk might make the difference.
Makes sense! Presumably these errors wouldn't show up in fixdisk as that came up clean?
Indeed, though maybe in some low-level SMART data. Also, a disk designed for desktop PC use may just not support the AV extensions.

This is somewhat hypothetical as I don't think we've ever definitively shown that the SATA AV extensions are used, though it would be consistent with the specification of the OEM disks.