RMA stands for Return Merchandise Authorization. Its a process you follow when you have a bad HDD.
The top four brands of 1TB Hard Drives are amongst others Western Digital hard drives, Seagate hard drives, Toshiba hard drives, and Hitachi hard drives.
Hard disk drives
How are Hard drives indentified and what are their functions
Assuming that when you say system unit of a computer you mean the computer case A.K.A. the tower, then no, hard drives are not always installed internally. Hard drives can be in external enclosures, hard drive docks, and even network attached devices.
Mac laptops may likely have solid state drives (not hard drives). Those drives are 2.5" form factor and can be replaced by hard drives.
Riley RMA was created in 1945.
If you mean between hard drives and ssd's (solid state drives), then you need to read the wikipedia page on Solid State Drives, it has a comparison of the two technologies.
Hard drives last for many years
There are tons of kinds of drives. Floppy Disk Drives, CD-ROM drives, PATA Hard drives, zip drives, flash drives, DVD drives, SATA hard drives.... The list could go on and on and on...
Simpletech produce portable external hard drives. These range from small USB sticks, to larger external hard drives. Simpletech also produces mini versions of the portable hard drives
RMA Gold Airways was created in 2004.
They store digital information in a relativly permenent form. Some hard drives called 'Hard Disc Drives' store data magnetically on a metal disc. Newer hard drives called 'Solid State Drives' store data electronically on flash memory.