2.5" externals, premade vs make your own

the WD passports are exactly what you are suggesting you will make. they are 2.5" laptop drives with a small sata to usb adapter. if you are going to carry them around i would go and buy a 128GB jump drive and just use that. or smaller if you dont need that much space.

As far as performance, no, but most or all current WD 2.5" externals contain a drive with electronics made specifically for USB, meaning there is no separate SATA-USB adapter board. IOW the drive cannot be removed from the enclosure and plugged into a SATA port.

I think that you are not going to save much money building the drive yourself.

External drives have become comodity items. Just buy the size you need.

Plastic external enclosures are hotboxes. Users often report temperatures of 50C or more.

In addition to the integrated USB bridge electronics on the HDD PCB, the Passports also incorporate AES hardware encryption, whether or not you have set a password. That may or may not be desirable.

If you build your own external drive, then you may have a longer warranty. Moreover, there will be no warranty issues if you need to remove the drive for any reason.