The proof is the market place. There are none. Do your own research. If you can find one. If you can afford one, buy it.
You can begin to confirm this by checking the websites of the hard drive manufacturers. The X-31 uses a "legacy" "IBM" drive 1.8". Try to find one that is larger than 40 GB and 7200 rpm.
Darned rare. Hitachi and Toshiba. IBM used them in the X3x series and the X4x series. For IBM there are no longer any 1.8" drives used in new models. There is now one 60 GB drive by Hitachi, and it is now 5400 rpm.
What better proof than you can no longer get them.
As far as I know, there is no longer any 1.8" drive at 7200 rpm, and the price of 1.8" drives with a capacity over 40 GB is close to $200. Even the 40 GB 1.8" drives are $150.
There are ZIF drives, IDE drives, Hitachi Legacy, and IBM legacy drives in 1.8"
If there was a market, which would be created by a reliable fast drive, you would see Fujitsu, Hitachi, IBM, Samsung, Seagate, Toshiba, Tri-Gem, Western Digital all selling them.
Your mileage may differ. Do you own search. Let us know if you find differently. Try Newegg, CDW, PCConnections, TechDepot, Frys, Outpost, PCMall, TigerDirect, Directron, Krex, ZipZoomFly, CompUSA, Circuit City, Best Buy, IBM, Lenovo, PCSupply, Cyberguys, You are looking for something that no longer exists.