No Identifiable Commonalities
Don't run Digital Vault and the software on each computer is different (except for different versions of the MS Office suite). ReadyBoost to a fast SD card has no effect (easily provable by removing/replacing the SD card).
Microsoft is definitely not taking this disabling anomaly seriously, despite the problem being widespread and long-standing. I've noted many opinions on forums that it's just classic MS arrogance. One thing I cannot understand is the ephemeral nature of this issue (i.e. some have it consistently, others intermittently and many others, not at all).
I've tried rebooting (rather than just hibernating) - but there's no apparent difference. Defragmenting makes no apparent difference, but I am also aware that the science of defragmentation has changed significantly. Diskeeper uses an intricate proprietary technology (based upon access stats), whereas Windows 7 defragmentation is based upon old school stuff (of just stacking files contiguously).
I've now diverted my attentions to finding the best alternative shell program (to Windows Explorer). Suggestions beyond Total Commander (also affected) would be welcome.
UNC
Edited to remark that the antivirus is intentionally different on each machine (Bitdefender, AVG and AVAST). There's also a Buffalo TeraStation NAS tied into the network.