For the Pegasus.
1. You can put in a drive that is the same size or larger. My guess is that the additional space will be unavailable.
2. The Pegasus manual suggest you can use any model you just have to meet these criteria:
Type – Hard disk drive (HDD) or solid state drive (SSD)
Interface - SATA 3 Gb/s or 6 Gb/s
Spin speed - 7,200 RPM or 15,000 RPM
Capacity – Equal size or larger
It specifically says:
You do not have to install the identical make and model of physical drive. However replacing with the same make and model of drive makes the process easier and simpler.
So it seems there is some flexibility in placing drives in the unit. The biggest question being how it handles putting in a drive that is larger if you plan to add it to an existing disk array. See nutshell section for how you can bypass this.
If a drive dies in the Pegasus, what occurs is going to depend on how your configured it.
1. If you used a fault tolerant RAID mode, your data will be fine.
2. You may have configured a spare drive, which means it will start rebuilding that drive to make up for the loss.
3. If you did one of the above, you should replace that drive as soon as possible. Promise covers your drives for two years, so you can request a replacement from them.
The Drobo handles everything for you. The rebuilding, RAID mode, etc. You don't really do anything. If you are unfamiliar with configuring RAID and the implications of doing it improperly or ignoring fault tolerance, the Drobo is a good bet.
The Promise can effectively do the same with their Automatic configuration mode.
The Drobo does have the any size drive advantage. However, this can amount to less available space overall as the Drobo is controlling the underlying structure. So while you can put in a 3TB drive, to maintain the same level of fault tolerance you may only be getting 50% of the space.
The Pegasus gives you much more control of your drives, you could put two larger drives in and place them in their own disk array and configure them however you like. Meaning you can get as much space you want depending on how much fault tolerance you require.
I have been using a DroboPro for over a year. My units power supply died and from there I had to wait from them to swap my unit. During that time my data was inaccessible without another Drobo. At the very least, with the Pegasus, I could get to my data if I used the same RAID controller. I also had tons of issues with Data Robotics RMA process.
At the end of the day, I feel more in control of my storage with the Pegasus whereas I feel powerless with the Drobo. The any size drive feature has little value when you consider you have no control over how the additional space is provisioned.
I would recommend looking over this:
http://www.promise.com/media_bank/Download Bank/Utility/Pegasus User Manual (English) .pdf
And this:
http://www.drobo.com/how-it-works/beyond-raid.php
Many of the features Drobo has, you aren't going to get. Such as thin-provisioning. But chances are, that stuff isn't even on your radar.
Bottom line:
If you want to set it, forget it and feel like your data is protected: The Drobo is better, but the Pegasus can also achieve this level of functionality.
If you want more control and speed: The Pegasus wins hands down.
Drobo of course has more than just DAS models, so if you are looking for NAS capability, the Peg is not an option.
