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The ReadData function already has 2 workarounds for some quirks with Sony and Panasonic drives, using READ_CD to emulate the behaviour of READ_DATA because the latter didn't work on the particular models Acorn shipped. However, with DVD drives some firmware vendors have been a bit literal about what READ_CD does, so you end up in a situation of being able to read a CD but not a DVD in an otherwise identical setup.
Change the retry loop from 2 goes to 3. On the 3rd attempt just try using READ_DATA(12) like the standard intended, so we retain the former workarounds and only use that command last. Note that READ_DATA(2) only has a 2 byte block count which would be a truncation from READ_CD's 3, so using READ_DATA(12) with a 4 byte block count keeps the command block shuffling simpler.
Tested on a HP branded TS-H653 and Pioneer DVR-TD11RS with an ISO9660 DVD-R.

Version 1.39. Tagged as 'ATAPI-1_39'