Does anybody know (Prodatron?) what sort of loading delays there are on the real Symbiface IDE interface, and if there's a maximum time allowed between reads of data from a sector? Would it be possible to interleave loading sector bytes with playing output through the Digiblaster or AmDrum?
Well, I did this in the 90ies already with the Dobbertin HD20. (See the first 6128 Plus demo ever, made my me back the day). Also with IDE devices you basically tell the device which sector you want - then you just read data when you want. I did not do studies over days, but as I know it doesn't matter if data remains in the buffer for a longer time. It will stay there until you force to read a new sector. However not reading all data will(!) give you an error. And it will take a bit to get the controller back on the good way.
Remember this is not a floppy disc, in such a case you have to read data when the FDC delivers. But a HDC keeps data in buffers, so to read and write is much more simple, ease and error prone compared to FDC management.