The TS-253D has a PCIe 2x4 slot, while the TS-453D has a 2x2. The TR-002 expansion unit has a SATA3 6Gbps controller, whereas the TR-004 only a has a SATA 2 3Gbps controller, or is that not correct? TR-002 has USB 3.2 Gen2 , TR-004 has USB 3.2 Gen1. I bought the TS-453D knowing the difference on the PCIe slot ahead of time, now reading the fine print on the expansion units. I haven't decided what to use my PCIe slot for. To me buying into old SATA 2 technology, PCie 2x2 USB 3.x Gen 1, DDR3, etc is undesirable. Is SATA2 ok? USB 3.2 Gen1 ok? Where is the performance bottleneck I should concern myself with? Why have a lesser spec on the 4 bay unit? to save a few bucks. I think it only leads to frustrated users that have researched the 2 bay unit but decided to up their budget. Call the 4 bay model something different but make the specs the same if using TS-x53D and TR-00x.
No answers for that question.