No luck recovering my lost files but at least this is encouraging - got an email from B and H related to the following review that I left on the Marantz 706 product page.
Here's the review -
Lots of features for the price - and a big glitch 4 stars out of 5
I use this machine for recording music in the field. First impressions - nice feature set, good in/out options, adequate build quality, easy to power, preamps sound good.
Now for the bad news - if you lose power (in my case the USB connection for the external battery came loose and it drained the AAs I had in for a backup) the file in progress will vanish. No recoverable data on the card. No file with a 0byte incomplete header like just about every other recorder on the market. Nothing.
I contacted Marantz Professional support after my incident in hopes to recover the 1h:20m of the concert I had been recording and was told quote - "that's the way it works - lose power lose files". Tested at home under same conditions and same results. No files.
Hopefully this can be fixed with a firmware update.
Otherwise make sure you have a good fit for the USB cable for your external battery (I was using the supplied cable but not anymore) and fresh AAs in addition to checking the battery icon (it will show which power source - USB disconnect automatically defaults to internal batteries) to ensure you have plenty of power.
And the email from the B and H Product Review Team -
The B&H Product Review Team took notice of your recent review describing the issues you experienced with the Marantz Professional 6-Channel DSLR Recorder with up to 96 kHz/24-Bit Resolution (Review # 99253217) and brought it to the attention of Marantz. Like B&H Photo, the overwhelming majority of our suppliers truly care about customer satisfaction with their products. To this end, Marantz has asked me to reach out to you, as they would like to let you know that they will have a firmware update soon to correct the auto-save feature.