ZX Spectrum disk interfaces

A question born only of idle curiosity. In thge 1980s, I bought an MGT +D disk interface for the Sinclair ZX Spectrum: https://en.wikipedia.org/wiki/%2BD In the end, I traded mine in for its upmarket cousin, the DISCiPLE: https://en.wikipedia.org/wiki/DISCiPLE (I link to WP as I originally wrote those articles, IIRC. :-) ) I have noticed that multiple people have now cloned the +D: http://sintech-shop.co.uk/sinclair/-d-disk-interface-clone/a-6196/ http://www.rwapsoftware.co.uk/spectrum/spectrum_storage.html#plusd But this post, from FB, claims that the Beta Disk interface can handle high-density drives, giving 1.5MB per disk: https://www.facebook.com/groups/speccy4ever/permalink/1237322356346334/ Given that HD floppies and drives are far more readily-available than DD these days, how come most 8-bit interfaces can only handle DD? Is it purely a data rate issue? -- Liam Proven ? Profile: http://lproven.livejournal.com/profile Email: lproven at cix.co.uk ? GMail/G+/Twitter/Flickr/Facebook: lproven MSN: lproven at hotmail.com ? Skype/AIM/Yahoo/LinkedIn: liamproven Cell/Mobiles: +44 7939-087884 (UK) ? +420 702 829 053 (?R)