I had the same problem for a while and it turned out that a failed harddrive was (probably) the reason. Once I exchanged the drive, the installer could reflash the TS. But: a word of warning: it absolutely wanted to reformat all my drives (not just the new one). So it might be wise to recover your data before trying to reflash the TS. See here for the details: http://buffalo.nas-central.org/wiki/Data_Recovery
(I did it with UFS Explorer, which is by far the easyest method if you don't have a Power PC at hand). Someone who is very confident in their mdadm knowledge could also try the --update=byteorder switch.
Update the superblock on each device while assembling the array.
The argument given to this flag can be one of sparc2.2, sum‐
maries, uuid, name, homehost, resync, byteorder, devicesize, or
The byteorder option allows arrays to be moved between machines
with different byte-order. When assembling such an array for
the first time after a move, giving --update=byteorder will
cause mdadm to expect superblocks to have their byteorder
reversed, and will correct that order before assembling the
array. This is only valid with original (Version 0.90)
I have not tried it yet (first need to copy over the images, which takes some time).