Make sure your Atari has a 1.44MB floppy drive, not a 720KB drive. I've heard that some of the older TT030 have 720KB drives.
Hard Drives Edit
Your TT030 probably came with one internal SCSI drive. Obtain a second drive onto which you'll install NetBSD. Why? First of all, you'll probably want something a little more modern (and quiet!) than what's in your TT030 to start with. Second, dealing with multiple partitions with NetBSD / TOS, while possible, is difficult and prone to failure. It's best just to dedicate a new drive to NetBSD and then store the old TOS drive somewhere safe when you're done with it.
Recommended Drives Edit
I currently use a 36.4GB IBM 90P1312 Ultra320 10K 2.5" SCA drive attached to a Corporate Systems Center SCA2SCSIT adapter. Ultra320 drives are backwards compatible all the way down to SCSI-1, in theory, though in practice I've found that inexpensive converters don't work reliably. (Even the ones that are advertised to provide correct high-byte termination, etc.) The SCA2SCSIT / 90P1312 combination works like a charm and is super reliable.
One minor issue with the SCA2SCSIT is that the drive activity LED is directly mounted to the board. I'm not great with a soldering iron, but I was still able to break off the existing LED on the board and solder on my own tailed green LED to illuminate the drive light on the Atari. The IBM drive is quite fast and supports SCSI linked commands, unlike the Adtron S35H solution below. While the drive itself is warm to the touch when disconnected, standard metal 2.5" to 3.5" drive rails act as a heat sink and allow the drive to run very cool.
Previously I had been using an Adtron S35H purchased from SCSI for Samplers. The S35H is attached to a standard notebook IDE drive. The Adtron S35H unfortunately does not support linked commands and it has a slightly annoying 9GB cap. You need to ensure that the drive installed in the S35H is larger than 9GB, and also be prepared to forgo any extra capacity above 9GB. (Effectively the 9GB cap means that the drive reports a fixed size of 9GB, no matter what.) But overall it was a better option for me than the Seagate drive described below because there are no issues with generated heat. I've had absolutely no reliability issues with the Adtron device.
A final option is the Seagate ST318418N. It's a 7200 RPM drive so it runs a little hot, but I've found that it's stable as long as it has sufficient cooling. (When it was sitting on top of my TT030 as I was building Samba for several days, I wound up having a consistent SCSI related kernel panics. This all went away when I aimed a fan at the drive. It also seems that the drive is cool enough within my TT030.)
NOT Recommended Drive Edit
Previously I had attempted to use an MTRON IDE SSD with an ACARD SCSI to IDE bridge. Unfortunately this setup does not support SCSI linked commands (as reported by the kernel upon boot), but worse it causes frequent kernel panics on NetBSD 1.6.1 (different from the heat issues I was seeing with the Seagate). So although the ACARD bridge may look attractive, stay away!
Installing the drives in preparation for OS installation Edit
As you may be aware, each end of your SCSI bus should be terminated. Before starting off with your install, check to see that the terminating resistors are in place on the motherboard. If they aren't, and you can't find them, I would suggest installing a SCSI terminator on the external SCSI port at the back of the TT. The other end of the SCSI bus should be terminated by the internal drive at the end of the chain. Find a 50-pin SCSI cable with at least three connectors. The first connector should go on the internal SCSI connector in your TT. The second should be hooked up to the original Atari drive, which should not be terminated. The third should be hooked up to the new drive, which should have termination in place. Set the SCSI ID for the original drive to 1 and the SCSI ID for the new drive to 0. This way you can remove the original drive from the chain and the bus will still be properly terminated. Remember to temporarily re-terminate the original Atari drive as necessary if you decide to image it on a PC with a SCSI card or if you plug it in to the Atari without your new drive at the end of the SCSI chain. (In reality, improperly terminated SCSI chains actually do work quite often in practice, but still.)
HD LED Edit
When I originally opened up my TT030 to check out the installed hard drive, the HD LED crumbled in my hands. If the same thing happens to you, you can purchase a new green tailed LED here. If you remove the little plastic pieces on the ends of the cable, the exposed metal clips will fit on the LED pins of the Seagate ST318418N. It's not a great fit but you'll find it works well, especially once the LED cable is pressed against the HD casing when the HD is installed. To remove the plastic pieces on the LED cable, use a small pocketknife to lift up the little tabs, then they will just slide off. If you slightly bend back the piece of aluminum holding the old LED you can wedge the new LED into place so it stays put.