What does 3Ware's tw_cli mean by a "DEGRADED" disk vs "ECC-ERROR"?

ECC error means that there is at least one unreadable sector on the drive. However, if you are lucky, that sector might not actually be used by the filesystem located on that volume, therefore you might still be able to copy your data from the array in this state.

There are also some options to ignore ECC errors during rebuild:

/cx/ux start rebuild disk=p [ignoreECC]
/cx/ux set ignoreECC=on|off

However, using these options means that the RAID stripe affected by a bad sector will be corrupted (not sure what exactly the card will do in this case — it might replace the whole stripe with zeros, or even with random data), therefore the “recovered” array might actually have undetectable corruption (if the affected stripe was in the middle of some data file). Copying your data from the array to some other place before trying to rebuild might be safer (at least you should get errors when trying to read the bad area).

You should set up scheduled verify of the array to catch unreadable sectors earlier, so that you can replace a drive which just started going bad.

I have never experienced a physical drive (p0) to go into status DEGRADED, however you might be able to get back the ECC-ERROR drive or even the DEGRADED drive by removing them via

/c0 p1 remove

and then issuing a rescan

/c0 rescan

put them back into the raid unit via

maint rebuild c0 u0 p1

SATA-Drives that failed me with ECC-ERROR i was able to resurrect if even just for a few hours before failing again.

It's very likely your data is gone. ECC error means an unrecoverable error while reading from this disk.

If you haven't a backup, you can try to dump the current state of the array. This might be possible because the controller doesn't know if it lost data or just an empty area (it lacks any insight into the file system).