Oh wow... So in theory I could run 4no 2.5" USB connected HDD's and 5no 2.5" SATA HDD's at the same time!As I guessed might happen, Radxa has just released a new Penta SATA Hat that can be used on their Rock Pi 4 and Rock Pi 5, and on a Raspberry Pi 5. This provides 4 conventional SATA connectors (including power), plus an eSATA connector, all running off of the M.2 PCIe available on those various SBCs. See Penta SATA Hat. $49.
Out of interest... What file systems are supported? Currently all my 2.5" USB connected HDD's are formatted to NTFS. This is useful because I often unplug them from my Synology NAS and plug them into other devices USB ports, such as TV's, my OPPO's and my Windows computers (to transfer media files more quickly).
Cheers