User can absolutely resize partition on computer, but how about resizing external hard drive. External hard drive generally includes USB mass storage or mobile HDD, etc. To resize such external hard drive, user needs a third-party partition manager, such as MiniTool Partition Wizard.
On external hard drive:
An external hard disk drive is a type of hard disk drive which is connected to a computer by a USB cable or other means. Modern entries into the market consist of standard SATA, IDE, or SCSI hard drives in portable disk enclosures with USB, eSATAp, eSATA, SCSI, IEEE 1394 Firewire client interfaces to connect to the host computer.
The internal structure of external hard disk drives is similar to normal hard disk drives; in fact, they include a normal hard disk drive which is mounted in a disk enclosure. In a 2009 Computer Shopper comparison of 5 top external hard drives, the capacities ranged from 160 GB to 4TB and the cost per gigabyte value varied between ~0.16-0.38 USD. Larger models often include full-sized 3.5" PATA or SATA desktop hard drives, are available in the same size ranges, and generally carry a similar cost. More pricey models, especially drives with biometric security or multiple interfaces, generally cost considerably more per gigabyte. Smaller, portable 2.5" drives intended for laptop and embedded devices are slightly more expensive in cost per GB compared with larger capacity 3.5" drives. Small MP3 players, previously built around mechanical hard drive technology are now primarily solid state CompactFlash based devices.