It was a Logitech USB 2.0 M/F cable extension (#501392-0000) identical to this one (pic found on eBay): I would assume the 4-pin USB 2.0 were just extended but somehow it prevented the HDD USB case from booting and getting recognized by the DMP-A6. I suppose the 9-pin USB 3.0 is required, the DMP-A6 might not be able to default to USB 2.0 and only supports USB 3.0?
I have a USB3 version of that cable type 1m long coming in today and will report back if there are any issues connecting an external SSD.
So to that extension cable test. I purchased a 1m USB3 UGREEN extension cable from Amazon, it is male A on one end and female A on the other end, OTG style. Connected into the top OTG port of the A6, OTG setting set to mass storage. I tested two external drives connected with this extension cable. First a Sandisk 1Tb Extreme SSD, then a Western Digital My Passport Ultra 2Tb HDD, both take power from the USB bus. A short USB3 30 cm C to A cable was used to connect the Sandisk to the extension cord. The WD disk had its own USB3 custom to A cable, again about 30 cm long. Result is that both drives are detected fine and list correctly under Files. I can play tracks directly from them and all indications are normal on both the device and control App. So success all around. This is just the same as if I had used my normal powered USB3 hub with either of these drives. So these drives do not seem to pull excessive current from the A6. Found an old 3m USB2 cable with male A on each end. Using a USB female A to male C adapter, I could then connect the Sandisk SSD to the A6. This also worked fine, completely normal detection of drive, music playback fine, no issues. So even a long and type 2 cable did not cause any issues. Could not test the WD drive with this cable as it has a custom non C type connection at the device end. So the A6 behaves more permissively with these external drives at least than I had imagined before the test. If your external drive will not power correctly or cycles power on/off when connected to the A6, I would say this is an indication it is drawing too much current. Either use a different drive with it's own power feed or use a powered USB3 hub to connect. Hope this helps.
The drive works normally when directly connected to the USB3.0 port. Only adding the cable extension breaks it, and I cannot imagine a 1-meter cable pumping lots of power, if any at all. So, mileage may vary, but I would recommend a USB3.0 cable just to be safe.
