Parallel ATA a.k.a. IDE, ATA, ATAPI, UDMA and PATA — legacy, wide 40-pin connector for disks produced few years ago. In case of notebook drives pins are smaller, and there is also power supply in the same plug.
Serial ATA (SATA) — modern connector. Most modern laptops use it. 6 data pins
and 15 pins including power supply.
Sizes of laptop disks:
- 2.5" — most common
- 1.8" — reduced, used mostly in ultralights and netbooks.
- 7200RPM — modern, high-end 2.5" disks. Consume more energy.
- 5400RPM — standard, low-end 2.5"disks or high-end 1.8"disks. More energy-efficient.
- 4200RPM — legacy, low-end 2.5" disks, some modern reduced height and energy-efficient 2,5" or standard 1.8"disks.
Take in account, that 5400RPM HDD with bigger capacity actually might have faster transfer rates, than smaller capacity 7200RPM. Rotation speed does however directly affect seek times.
HDD vs. SSD:
Modern alternative to mechanical HDDs are Solid State Drives, based on flash memory. They have almost instantaneous seek times, incredible read speeds and very low power consumption. As of now they are still much lower capacity than similarly priced HDDs. However, with the effects of Thailand floods and sharp drop in SSD prices in recent years, there are no longer excessively expensive. SSD can come in 2.5" SATA form factor, thus be interchangeable with 2.5" HDD. Another form factor, unique to SSDs is Mini-SATA (mSATA), intended mostly for use with netbooks (and some ultraportables). Below mSATA drive on top of 2.5" SATA HDD for size comparison:
Note, that ultrabooks use neither of these formats. In ultrabooks SSDs are soldered permanently onto motherboard, thus cannot be removed nor upgraded.