The [[Atari ST]] was released in 1985. The case looks a lot like the 8bit Atari XE. It originally had an external PSU and an external single-sided (360KB) 3.5inch floppy drive. That resulted in games using single-sided 3.5inch floppies for a long time, even supplying two 360KB floppies instead of a single double-sided one. Also, the TOS had to be booted up from floppy disk.
In 1986, the Atari STF put the TOS in ROM and integrated the PSU and a double-sided floppy drive inside the machine itself. It was sold in 512KB and 1024KB configurations.
A version intended for business users was also created in 1987 and named the Mega ST. It featured more RAM, a real time clock, a blitter chip for faster rendering and a detached keyboard.