I recently asked about upgrading to SATA III from II and was informed it was a waste of time. I've subsequently decided to go down the RAID0 path and just had a couple of questions I was hoping someone could help me with;
1. Will I need the exact same HDD as before to set up RAID0, including manufacturer, size etc?
2. Will data already existing on one drive be partitioned across both and become faster to access or will only files I save after setting up RAID0 that'll gain the benefit?
In creating a RAID array it is always best to use matched sets (in this case, 2) of HDD units if possible. If you only mismatch sizes, the RAID array will be limited to using identical spaces on each component HDD. If you mismatch other parameters like head seek time, RPM, or disk cache size, the resulting array can only use the slowest result and you lose some of the very small advantage of RAID0.
Before proceeding I urge you to find and read real performance measurements of RAID0 versus stand-alone HDD units. Look around Tom's for reviews that actually measure disk system speeds for many types of access. Do not rely on stories that suggest that ALL RAID0 systems are known to be much faster than ANY HDD by itself. Also read up on why RAID0 is more risky - more likely to result in TOTAL data loss, and hence requires more careful attention to a reliable backup system. Then make your choice whether to proceed.
Few systems that create RAID0 can migrate existing data to the new array. And yes, normally ALL of your data will be transferred to the RAID0 array. By far the only really reliable way to do this is through backups. Make a backup of ALL of your data, etc. Then VERIFY that your backup is good so that you can restore from it. If you want to be really secure, make a second backup.
If you plan to have the RAID0 array as your only "disk" you will have to install the OS on it and boot from it. The dilemma is that most OS's (including Windows) do not know how to use the exact version of RAID you have. So you MUST install a RAID driver as you RE-INSTALL your OS on the newly-created RAID0 array. Read your mobo manual for how this is done - it varies, depending on which OS or Windows version you have. This is necessary so that Windows (or whatever) can find the right driver at boot time to be able to use this oddly-organized "disk" that is really two HDD's with data spread over both, and hence basically to find and load all its own components. Once the re-install is done you can restore all your data and applications from your backup. ALL your files will be on the RAID0 array, which Windows will "see" as ONE "drive" even though it consists of two HDD units.