Useful programms are hdparm, dmesg, fsck and fdisk .
Be careful when using your laptop abroad. I have heard about some destroyed harddisks due to a magnetic field emitted from the magnetic-holds at the backresttable of the seats in a german railway waggon.
Though I am quite satisfied with the quality of the harddisk in my laptop, when I removed it from the case I unintendedly dropped it, I recommend to be very careful.
AFAIK there is only one form factor for harddisks used in laptops the 2.5" format. This format seems to be available in different heights (Please note I couldn't verify this information yet):
18mm: laptops build before 1996 usually have drives 18mm high
12.7mm: I got a report about such disks but without a notebook model or manufacturer name
11mm: since 1996 the drives are 11mm high
9mm: many laptops, including the subnotebooks, now use a 9mm-high disk drive. The largest available in this format in late 1999 is IBM™ 12GN.
9.5mm: Toshiba Libretto L70 and L100 have a 9.5mm HD
8.45mm: Toshiba Libretto 20, 30, 50 and 60 have 8.45mm tall HDs
6.35mm: Toshiba Libretto L1000 has a 6.35mm HD
It might be possible to use a hard disk wich doesn't fit with some case modifications.
Some laptops come with a removable hard disk in a tray, for instance the KAPOK 9600D. There seem to be no SCSI drives for laptops available.