If there is a 4TB hard drive that costs $125, a 1TB hard drive that costs $29.99 and a 2TB hard drive that costs $60, which one would be more cost effective, and is it worth to buy compared to the other one? Is it as reliable and what else should I consider?
As a language model AI, I don't have personal preferences or opinions but I can provide you with some information that can help you make a decision.

When it comes to cost effectiveness, it is best to look at the price per terabyte.

For the 4TB hard drive, the price per terabyte is $125 / 4 = $31.25.
For the 1TB hard drive, the price per terabyte is $29.99.
For the 2TB hard drive, the price per terabyte is $60 / 2 = $30.

Based on the price per terabyte, the 1TB hard drive is the most cost-effective option. However, if you need more storage space, the 2TB hard drive may be a better option for you. Or if you prefer more storage in a unique hard drive, the 4TB hard drive is your option. 

When it comes to reliability, it's best to do some research on the specific brand and model of each hard drive. Read reviews and look for information on the warranty and customer support. Additionally, it's important to consider the intended use of the hard drive. If you're using it for important data or critical applications, it's best to opt for a hard drive with a good reputation for reliability and durability.

Finally, you should also consider the interface and speed of the hard drive. SATA and SAS are two common interfaces that are used in desktop and server computers, respectively. You should choose a hard drive with a compatible interface. Additionally, consider the read/write speed of the hard drive and whether it meets your performance requirements.
Does it work if I have multiple drives? Like a big one and a small one?
If these are external drives and you don't need to store a single file that exceeds the capacity of the smaller drive- yes. If these are internal drives then yes, assuming your motherboard support multiple drives.