Hi,
recently I got a IIfx on you-know-who-bay and replaced the internal HDD with a 9GB IBM DNES UW. It flies... but not as fast as I expected.
Unfortunately I have no experience with old macs and I've come to wonder if it's the bus itself. I just can't seem to get more than 700KB/s out of the drive 
(Benchmarking done with FWB 1.7.5)
What am I missing?
You'll need a NuBus SCSI card to get some real speed out of the drive. The JackHammer seems to be a good choice.
So its true that the internal SCSI of the IIfx won't dish out more than 700kb/s?
The IIfx requires a specific SCSI terminator to work right. It does have a custom DMA setup that should be faster than other II series Macs, but for the price of a JackHammer card, you'd likely save a lot of trouble and it'd be faster in the end. The JackHammer cards have both 50-pin narrow and 68-pin wide SCSI plugs. You just aren't going to get fast speeds out of the 50-pin IIfx port without it running in a Fast-SCSI mode. The JackHammer will at least use the drive in Wide mode, even if it might not suppor the Ultra mode.
Proper termination might speed up the SCSI bus, but it also needs to have MacOS aware of the DMA capabilties and use them. See links from http://www.lowendmac.com/ii/iifx.shtml regarding the SCSI DMA and term issues.
Thanks - I will take a closer look at that technote and see if I can speed things up.
What does this special 'black' terminator look like? Is it a centronics Terminator in black (I have one of those). If I remember correctly these were also calles 'active' terminators?!
Thanks.
Yep, the external chain needs to be terminated with the special black centronics active terminator. There's also a small, thin, special black or gray plastic coupler that fits into the internal motherboard 50pin SCSI port. I don't remember if that was a terminator or not, and if so, why it would be at that junction in the chain, but some IIfx motherboards have them and some don't. Maybe someone else knows what that was for.
And, this old Apple TechNote mentions a bit about the IIfx SCSI speeds. Of course statement that the IIfx has the fastest SCSi bus is wrong at this point in time, but it does say that you won't get better than 3megabit/sec out of the IIfx SCSI. The NuBus has the capability of 10Mbps, so if you attach a Wide SCSI (68-pin) capable card, or even a Fast SCSI capable card, it should be to beat the stock SCSI hands down. If the card supports Wide-Fast SCSI it'll saturate the NuBus bandwidth. I'm not sure if there are Ultra capable NuBus SCSI cards (I've not looked) but even an Ultra Narrow (50-pin) should be overkill.