[Solved] WD MyBook Live Duo / two disks

The interesting part is not where it ends, but what happens in between. I compared it to a bootlog of two hard disks, and I find one interesting difference already before booting:

supports DRM functions and may not be fully accessable.

And later on, for each drive:

[    0.782882] sata-dwc 4bffd1800.sata: sata_dwc_isr: DMA not pending intpr=0x80000080 status=0x00000050 pending=0
[    0.782917] sata-dwc 4bffd1800.sata: sata_dwc_clear_dmacr DMA protocol RX and TX DMA not pending tag=0x1f pending=0 dmacr: 0x00000006
[    0.782923] sata-dwc 4bffd1800.sata: DMA not pending tag=0x1f pending=0
[    0.782931] sata-dwc 4bffd1800.sata: sata_dwc_dma_xfer_complete DMA protocol RX and TX DMA not pending dmacr: 0x00000004
[    0.822456] sata-dwc 4bffd1800.sata: sata_dwc_isr: DMA not pending intpr=0x80000080 status=0x00000050 pending=0
[    0.832525] sata-dwc 4bffd1800.sata: sata_dwc_clear_dmacr DMA protocol RX and TX DMA not pending tag=0x1f pending=0 dmacr: 0x00000006
[    0.844468] sata-dwc 4bffd1800.sata: DMA not pending tag=0x1f pending=0
[    0.851052] sata-dwc 4bffd1800.sata: sata_dwc_dma_xfer_complete DMA protocol RX and TX DMA not pending dmacr: 0x00000004
[    0.861976] sata-dwc 4bffd1800.sata: sata_dwc_isr: DMA not pending intpr=0x80000080 status=0x00000050 pending=0
[    0.872042] sata-dwc 4bffd1800.sata: sata_dwc_clear_dmacr DMA protocol RX and TX DMA not pending tag=0x1f pending=0 dmacr: 0x00000006
[    0.883988] sata-dwc 4bffd1800.sata: DMA not pending tag=0x1f pending=0
[    0.890572] sata-dwc 4bffd1800.sata: sata_dwc_dma_xfer_complete DMA protocol RX and TX DMA not pending dmacr: 0x00000004

These are errors.

It looks to me like OpenWrt can't properly deal with (these) SSDs on SATA, or something is wrong/off with them. Someone more knowledgeable than me will have to chime in, I'm out of my depth here.