This is correct. They are like super slow SSDs in this respect, as they do a read, modify, write on existing data. It also means that they treat the SMR section as a "data pool" and its not actually physically mapped as a normal drive. So when you write a new file, it just finds the next free section in the pool and writes it. When you modify a file, it reads from the SMR section, modifies it, and then writes it back to a clear section of the SMR area. Once the drive is filled with data though, where a "TRIM" would be useful, they go into read modify write on the same tracks, and that's where they can get very slow, as they have to read the entire track, modify it, and then write the entire track back again, even for a 1 byte change.