Buy one drive that meets your needs for the 6-12 months. Because prices drop, you can buy more later at a lower price as needed.
Let’s start by assuming you are considering SATA internal drives. These are cheap and fast.
Disadvantages of 2 X 500Gb CONS.
They have two of everything. The redundancy of moving parts, comes at price.
• Cost more $$ (From PriceWatch 2X40= $80 vs $65 for a 1TB)
• Internal drives consume 2 connections on a motherboard and 2 bays inside a case. This may not be a problem if you have extra bays, or impossible if you are down to your last bay. An add-on card may be used to add SATA ports, if you have a spare card slot.
• Consume more power, which may push you over the limit of your power supply.
• Generate more heat.
• Make more noise.
• 2 drives of the same quality (MTBF) will fail twice as often, but not at the same time. The single drive 1Tb could fail destroying all of your data. Fortunately, sudden complete failure is (in my experience) rare. Usually drives develop symptoms that you should learn about. Check the Error Log, Consider using SpinRite or other diagnostic tool.
Advantages of 2 x 500Gb
Why would anyone ever buy 2 x 500 Gb.
• Incremental outlay. You can buy one now and one later. Although you will probably not buy another 500gb drive.
• RAID. Redundant array of independent disks by definition require multiple disks.
• Data Safety RAID’s are typically to protect data against real time disk failure or to ensure high (e.g. 24X7) availability. Usually all of the drives are the same size, so that a defective one can be quickly swapped with a spare which is the same size.
• This is a common requirement for large businesses, but I am guessing not a goal for you. Most end users get by with occasional backups, where data created since the last backup may be lost entirely. This is an acceptable tradeoff of money vs. safety.
• Speed. Two drives offer two controllers, buffers, and 2X heads, which in some circumstances results in less overall latency (seek time). The improvement is not 2X because often all of the activity is concentrated on one drive. E.g. all of your music on one drive, all of your pictures on another. In this case there will be little if any speed up.
• It is possible to use a RAID for striping, where at the lowest level individual files are written across two or more drives. Note this puts all of your data risk. If either drive fails, you may not be able to recover any of your data.
Finally, two external drives are more cumbersome, unless you only need to carry one at a time, or this is a desktop setup.