Bad eraseblock

I have the following entries in the log:

[    2.550000] Scanning device for bad blocks
[    2.570000] Bad eraseblock 56 at 0x000000700000

What's a bad eraseblock? Is it HW/SW, does it matter and can it be fixed?

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A bad erase block is a block on the flash chip which is broken and does not reliably retain data anymore (a previous attempt to write and read-back data from it failed or yielded differing results). Such a block gets marked as failed so that the higher software layers such as UBIFS can "navigate around" it.

It is expected for some regions of a NAND flash chip to eventually fail, sometimes blocks are already bad from the factory. The message you see indicates that one such broken block was identified and disabled.

As long as you don't see dozens of these messages with different addresses you should be good.

Well, few bad eraseblocks is normal for NANDs
A NAND can have bad erase-block even if your kernel does not show them. What it display is just a list stored in the NAND itself to list known bad blocks. I currently have never seen a NAND chip with zero bad block.