"I have went through boards and it appears that when the SSD and a Normal HDD is set in RAID mode in Bios that the system configures the SSD as a cache drive."

I don't know where you read that but it is not true. The system does not configure anything useless you do it. You can have the SSD be the boot drive and use the HDD for additional store.

I don't know enough to answer your other questions.

VIC384

Thank you for answering none of my questions and pointing out that the PC will not do anything without correct input from the idiot (Me). That would be the reason I joined the forum to get input from users much more versed in the Dell XPS 8910 which has not even been delivered to me yet.

I apologize if I offended you. You stated "HDD is set in RAID mode in Bios that the system configures the SSD as a cache drive". I took that to mean if you set SATA Operation in the BIOS to RAID that somehow the "system" configures the SSD. I didn't count you as part of the "system", by "system" I thought you meant the BIOS or Intel Rapid Storage Technology (RST) and somehow this was an automatic result of setting RAID mode. .

I have a Dell XPS 8910. I have has it over two years. I have added two SSDs to it along with the original HDD that came with it.

Perhaps I didn't answer your questions, but I wanted to correct the statement about what may have appeared on the message boards.

Is the 512 GB SSD going to be the boot drive? In that case, why would you even need a cache drive? Typical cache SSDs are 32 GB and cache a HDD for extra speed. A boot SSD likely has all the speed you need without a cache.

Install the OS and your apps on that 512 GB SSD and use the 2 hard drives for storage.

A question you're going to need answered is if that SSD is the boot drive, can you still set up the 2 HDD data drives in RAID1.

Why not just install some backup software that automatically backs up (images) one of the two HDDs onto the other one on a set schedule so you don't have to think about it? Might be easier than the configuration you're proposing...

Here is the answer to two of your questions:

"Does the SSD cache drive have to be in the M2 slot or can it be just any conventional SSD drive?" SSD caching is not restricted to M.2 or SATA SSDs.

"Can the cache drive be partitioned to load OS and function as cache drive?" According the this site: https://www.urtech.ca/2018/01/solved-install-ssd-caching-using-intel-rapid-storage-technology-rst/

a SSD caching prerequisite is that the OS MUST be installed on the spinning disk and the SSD should be EMPTY, not even formatted or partitioned. The video on the site showed only a maximum of 64GB on the SSD can be used for caching, the rest can be partitioned and used for data storage. So SSD caching may be worthwhile if you have a very small SSD, but with a SSD large enough to support the OS and more there appears to be no advantage.

I like the idea of the SSD just for boot and make the 2 - HDD's for storage. I would however like the 2 HDD as a RAID 1 array even if a 3rd party Raid controller is required as the backup solution can be problematic. I am building this system for my brother in law and he has a tendency to play around and break stuff. He is using this rig solely for Music and pictures. Did I mention I live 1800 miles away from him so eyes on PC if issues arrive will be by teamviewer only. A Raid 1 array would ease his mind (as well as mine), I could  figure out how to make sure Windows writes for all documents, music and videos would automatically go to Raid array drives. This way should he mess something up with Windows all I would need to do is get a reimaged windows 10 boot image restored to a new SSD and send it to him and he wouldn't have to restore the 2 TB plus of music and pictures he already has.

Vic384

I agree the caching drive doesn't make as much sense now. The only benefit is if I used it and built a Raid 1 array and allowed Windows OS to install on the Array as well as content. I would then have to get a 3rd 4 TB drive and tell him to replace one of the Raid drives every 30 days or so and put the one he just pulled in a safe location until next month. I am using 4TB Seagate drives that are 7200 RPM with 256MB cache so Raid array performance should be acceptable with a 64 GB cache for the Raid array..

@I didnt do it  I am not very familiar with RAID 1 but I did some reading and I understand that one disadvantage is that since RAID 1 mirrors the drives it also mirrors the errors. For example, if you accidentally delete a file it will be delete on the mirrored drive also. So unless you have a separate backup strategy I think an automatic scheduled backups as @RoHe suggested is better. With automatic backups you may wish to also do incremental backups so that work in progress does not get lost.

Vic384

That is the main weakness of having the OS on the same drive as the Raid 1 Array. Ideally I would have OS on a different drive (SSD) and the Storage drive would be my Raid 1 array. The biggest advantage of Raid 1 would be instant backups. Rebuilding an OS drive would be relatively minor compared to restoring all the music and pictures. However 4 TB drives are 100 bucks and as long as he faithfully pulled and replaced one drive in the array each month or so he would always have a true restore that would never be less than a month old. If he did it on a more frequent schedule that would be even better. Should the system get corrupted all he would do is pull both drives and boot with the backup drive. Once the system is up he would put in one of the corrupted drives and it would mirror to the operating drive that he had just inserted. It would be nice to have a drive tray that had hot swap hard drives but not possible unless I could afford to get him all SSD storage as there is only one 5.25 bay on the system and that would require giving up DVD drive. There are 4 slot hot swap SSD cages with trays that only take 1 5.25 slot but it takes at least 2 5.25 slots for cages for systems with 3.5 drives.

@I didnt do it  There are lots of articles which says that RAID is not a backup substitute. Here is one such article: https://www.backupassist.com/blog/support/raid-or-backup-is-raid-a-good-backup-substitute/

Your plan to have another hard drive which serves as a backup and is updated once a month or more does mitigate the problem but you could still lose a month of work or whatever work that was done since the last backup. The big pro for RAID is the ability to protect your data in real time. You just have to weigh the pros and cons and determine which solution is best for your situation.