As per my previous issue with using SATA USB converters this issue happens sometime after a high data (70GB, 8hr) flow starts not always/ever at the same point in either data or time, it is inconsistent - sometime the enstire transfer works, other times not. The difference is that after the event the drive returns/reconnects as a valid accessible device whereas it did not before.
The USB SATA converter I'm using now is not AC powered, it 2 USB leads and I'm connecting it through a 4 port (powered) hub. The USB cable between the HUB and Pi has had the power lines cut so there is definitely no power back feed issue.
The 4 port hub also has a Ralink WiFi adapter in and it is wholly unaffected by any issues raised here.
To test this I just use the command
dd if=/dev/sda1 of=/dev/null
A success is where 70GB are read without issue, failures manifest as shown on the right.
I noticed that if there was some consistency about where I plug the USB cables and the issue.
Easy to blame the Hub, but the Ralink was unaffected.
I should point out that in doing these tests I was plugging and un plugging and not being consistent about which 2 of the HUB USB ports I used for Data/Power.
Doing the test again (consistently choosing the same port for each) I got these results from the dd before it died:
Swapping the connections around these are the figures (and I killed the DD, it was still running).
It would seem there is some sort of a fault on one port on my USB HUB. It's not dead, but the results definitely port toward a port that is the cause of the problem.
I've ordered a new USB HUB and until it arrives will simply not use that port for USB data!