The Spectrum +3's DOS is based on the disc functions in Locoscript the CPC's DOS is AMSDOS. The Spectrum's DOS was developed after the CPCs, so clearly they saw the weakness in the CPC's DOS design and improved it.
The Spectrum Technical wiki can be found here. This describes the Spectrum hardware in more depth:
http://scratchpad.wikia.com/wiki/ZX_Spectrum_technical_information
===Consequences of a Spectrum Port===