The Tyros 2 has internal flash memory, about 2MB-3MB so enough to save quite a lot of midis (probably hundreds), though its shared with other user files such as new styles, registrations etc. You can also plug in a USB memory stick, a USB floppy drive, even a USB external harddrive if you wish.
DIMM is extra memory which is only used for the custom voice feature. The Tyros 2, unlike Tyros 1 can load new waveform data for creating new voices, that is new sample sounds. Tyros 2 comes with 4MB of sample memory as standard, but it can be expanded to 1GB with computer-like DIMM memory. Unless you are planning to create lots of new custom voices with new waveform data, or until Yamaha offer more for users there is little point in adding extra DIMM memory.
However once you move into the realms of adding more voices, styles then you need to look at a way of storing the data other than the user memory in the Tyros 2 (its a bit too small for voice sample data). Also adding a harddisk allows you to use the digital recording features on Tyros 2, that is a digital tape recorder which enables you to record your own voice, the sounds of the Tyros 2, external sound sources and instruments and then overdub it as many times as you like.
You dont need either a harddisk or memory DIMM's to use the Tyros 2. The harddisk is necessary to use the digital audio recording features and it gives you built in massive data storage, without having to remember to carry round and plug in USB drives or memory sticks.
Hope this helps,
Simon
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