The [[Atari ST]] was released in 1985. Atari had intended to release the 130ST with 128KB of RAM and the 260ST with 256KB. However, the ST initially shipped without TOS in ROM and required booting TOS from floppy, taking 206KB RAM away from applications. The 130ST was never released and the 260ST was actually shipped with 512KB of RAM.
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 RAM configurations.