Not too long ago I bought a Seagate 250Gb 2.5" SATA2 notebook drive, intending to use it as an external hard drive in a USB2 enclosure - very handy for backups and lots of portable storage in a case that will fit in your pocket.
Unfortunately, Seagate's drives appear to be notoriously power-hungry when spinning up, drawing up to twice the current that other brands use. This means that the drive is completely useless when drawing its power from the USB bus - you'll need an external power supply to keep it stable.
For me, this means it'll go into one of my HTPCs; my Vista Media Centre is about to be upgraded to have run off a RAID 0 array of Seagate 160Gb 2.5" SATA2 drives, so I guess this one will go into the XP Media Centre. Hopefully it will contribute to reducing the amount of noise from the system. (Oh, and my laptop runs off a Seagate 160Gb 7200rpm SATA2 drive. Notice a pattern emerging?)
So, you have been warned. If you want to use a notebook hard drive in a portable enclosure, don't buy Seagate.