Raid0 increases Sequencial read/write performance.
.. How it works. Raid0 uses what are called strips, normally 64 K (16 clusters, or 128 sectors). Say you have a file that is 128 K, then half of the file is on one Hard drive and the other half is on the other drive. When the file is requested the two halfs can be read at the same time. This is great for large files such as 10 meg photos. But has little value for a OS + Program drive. The majority of these files are only on one drive and are spaced randomily.
PS HDDs deal with sectors/clusters - 512 Bytes = sector and 8 sectors normally equal a cluster (4K cluster is default but can be changed). This makes understanding strips used in raid 0 easy to understand. Default varys but is normally 64K or 128 K). SSDs use what is called a Page, may be wrong but I think a page is either 64K or 128 K
A single HDD failure renders ALL files Kapout.
Yes You could raid a 128 gig SSD with a 500 Gig HDD. BUT you LOOSE 372 gigs on the HDD as YOU can NOT use it. Performance is totally unpredictible. Files that are not split would be read at the speed of which drive it was on. For files that are split part of the file would be at the speed of the SSD and the rest of the file at the speed of th HDD.
DO NOT RECOMMEND.
For an OS + program drive, do not recommend raid0, as benifits are not that great, IE Sequencial performance is increased, but access time Is not improved and very little improve in the 4K random read/writes. Here, Sequencial is the LEAST important paramter, while 4 K Random are the most important. Also do not recommend trim, with two SSDs prefer using one as OS+ Progem disk and 2nd SSD as a "Working" storage drive.
Raid0 is still a good choice for Starage drive when required to work with Large file structures.