The scenario you describe has come up before - or at least very similar symptoms.
Note that Nandroid restore will fail because it uses standard Unix tools such as "rm" to clear filesystems, so if a partition will not mount because of a corruption issue, nandroid will fail. I suppose that the same thing is true of the /sbin/recovery utility running underneath the booted recovery kernel (but I have not read the source code to verify that it is attempting to "mount" the filesystems first - if it didn't do that, it would need to understand the raw format details of yaffs2, and I think that is a stretch).
Unfortunately the filesystem formatting tools provided by Amon_RA do not include tools for repairing the mtd (NAND flash) - they are for the SD card/ extN filesystems. It is my impression, however, that the "yaffs2" filesystem is "format free" - meaning that a clean (Flash memory) "yaffs2" filesystem is simply a bunch of zero'ed pages - no superblocks, or Inode lists, - none of that. This suggests that the equivalent of "dd if=/dev/zero of=/dev/mtd/mtdNNN bs=..." could "repair" a yaffs2 file system by simply wiping it... but let's try something a little less crude than that (see below).
I had one of the file systems in my phone in this state at one time and I was able to repair the problem by reflashing the Root ROM - otoh, XDA user "stick" tried this and it seemed to produce a permanent brick in his case, so I am reluctant to recommend you do that. (You might, however, want to perform the jcase "Flash any RUU" hack to the "misc" partition so that you have flexibility to apply any PB00IMG.zip file)
Because the "flash_image" tool (in /sbin/flash_image in Amon_RA) writes both boot images and yaffs2 image files to arbitrary mtd partitions (and raw binary files to "misc"!), there is a chance that it is merely the equivalent of "dd for the MTD device" - so that you could "repair" a corrupted yaffs2 filesystem by simply overwriting it with a valid
yaffs2 image file. The repair strategy here would be to:
- Unpack any PB00IMG.zip file and move the contents to a folder on the SD card. (Verify the md5sums of the files on the SD card before you use them - use this reference
- Use "flash_image" from Amon_RA to flash the corresponding image file for the offending ("won't mount") partition, e.g.
flash_image system /sdcard/unpacked-PB00IMG/system.img
If this succeeds, see if you can "mount /system".
PS Don't try flashing "system.img" using fastboot. However it is engineered (by the HTC bootloader) it will fail due to space issues. It is possible that the HTC bootloader uses the /cache partition to temporarily stage the file, which is only 130 MB compared to 159.5 MB for the /system partition - but whatever the explanation, the experimental result is that that on the Eris, you can not flash /system from fastboot. All the other partitions, no problem - but not the /system partition.