I'm going to try to mod the HDD to CF. When people tested, there weren't many high end cards and if so very expensive. I think usable CF for this project was released in 2012.
Nowadays there are super fast CF cards with very good random write speeds as SanDisk Extreme Pro UDMA6 and 7 and Toshiba Exceria Pro 1066x (both higher than 1MB/s)
The Hitachi C4K60 HDD is ATA/100. That equals to 660x CF (or UDMA 5)
Hitachi random write is 0.5MB/s I think
Only a few Compact Flash cards (professional grade) have fast random write
I already have 660x high end card or faster to test. Ordered an adapter. If works, that should increase the battery live to last a concert.
Note: I think what you guys may have missed is that quality of HDD/SSD is measured on 4k writes not sequential write.
with that said, that is perhaps why Industrial (SLC) works but MLC, often, doens't. Same reason why we need those industrial USB to run WindowsToGo.
As I mentioned, now there are a few Compact Flash that are higher than 0.7Mb/s 4k writes (what it needs to write DSD).
Perhaps the way the audio is save is in small increments and thus the 4k write is more important than sequential.
Now as for the statement that SLC writing DSD doesn't prolong battery while the TLC (writing PCM) does might have something to do with using DSD rather than the CF.
Maybe some of you guys that still have the SLC cards (that worked to record DSD) can do a benchmark to check the 4k write speeds to help supporting my theory.
I can only test when I get the adapter ...