just test the surface sector bad or not , read verify sector means not read into disk buffer on pcb, just check bad or not. and you can test 256 sectors or more each time ,but read cmd just read one sector each time ,if you care the data
. or readmulti read 16 sectors once, if you care the data.
So you also can use udma control to finish the test, it's all the same. and speed is great too.
all above:The two commands are nothing special