Replacing all drives in a Terastation
Managed to replace the 4 160 GB drives in my Terastation 0.6TB (HD-H0.6TGL/R5) with 4 300GB Maxtor drives. My setup was:
Original Drives - 4 x Western Digital WD 1600BB 160 GB
New Drives - 4 x Maxtor L01R300 300 GB
Firmware before change - Original Buffalo 2.10 (Japanese) 
Read firmware instructions  before flashing and remember to switch off firewall
Experience level required - should know about flashing firmware and the dangers of doing so. Hardware side - if you are comfortable changing a motherboard in a computer, then you are probably OK. This is quite a bit more complicated than replacing HDs in a normal computer.
This was all done with a WIRED Cardbus Trendnet Gigabit adaptor on my laptop connected directly to the TS, with manually assigned IP addresses on both the computer and the TS (no DHCP). According to the instructions, this should not be done through a wireless connection, and I wouldn't trust doing it through any router either. The first time I tried to flash the TS, the Wireless adapter on the latop was on (as well as the Gb adapter), and the fw utility didn't seem to like that (I think the TCP metric favored the Wireless). Leaving *only* the Gigabit adapter "on", with one direct cable connection to the TS, and disabling any other network adapter present on the computer appears to be the safest way to flash the fw. It probably can also be done through a Gigabit switch - but to be safe, I wouldn't trust that either.
Here are the steps I took:
1 - Flashed the TS with 2.10 (Japanese) firmware before change (was using RAID5 on 4 disks - original fw was 1.12)
2 - Copied the files I wanted to save from original Terastion disks to an external USB drive connected to the computer. Switched off everything and disconnected all cables.
3 - Opened Terastation and swapped drives according to TS PDF manual's disassembly instructions  (new drives were new and unformatted). Lots of screws - have to keep them carefully separated and tagged as to where they go since their sizes differ.
4 - Switched computer and TS back on. Connected TS to computer. It came up on Client utility showing no firmware present.
5 - Flashed with 2.10 (japanese) fw again - utility ran fine. One dialog box that hadn't come up on the first flash came up. I don't know what it said, but I chose the left button (yes) and it continued. Takes a while to complete, and the computer showed the gigabit adapter switching off and on momentarily a few times. I think a couple of other dialog boxes came up with only one button in them, and I clicked on it every time till the utility went back to its original screen. Some people have said they needed to restart the flashing a second time in order for it to work (apparently the box mentioned above is a warning about formatting the drives).
6 - Rebooted computer and TS (on the TS, by holding the front button for a few seconds and waiting for it to power down, and then powering up again).
7 - Went into the web interface for the TS using the client utility and deleted the newly created default "normal" spanned RAID (array1). The admin password for the TS remained the same as the one I had created before changing the drives.
8 - A page in Japanese comes up when trying to delete the array, with four numbers on it and an empty text box. In order to delete the array, one has to type in the 4 numbers in the empty box (it is obviously a confirmation screen trying to make sure you want to delete the array).
9 - Created a RAID5 array. Disk check ran for about 7 hours - no errors.
10 - Copied the files back from USB external drive to TS. Switched computer's firewall back on.
RAID5 partition is now 836.2 GB, so it is almost twice as much as before. All working fine so far (October 7, 2006).