I have 3 Physical disks on a RAID 5 and would like to add another HDD to it to expand storage

To add a new disk to the existing virtual disk you should not delete current configuration. Is new drive which you inserted is of same family(SAS/SATA) and type(HDD/SSD) of current drives on the virtual disk? What is the status of new virtual disk? Can you also ensure you have latest iDRAC, PERC and BIOS FW installed on the server before trying reconfigure? 

I assume you are trying reconfigure from iDRAC? Is reconfigure option available from PERC Configuration Utility or OMSA?

Is new drive which you inserted is of same family(SAS/SATA) and type(HDD/SSD) of current drives on the virtual disk?

A: Yes, It's the same size SAS drive.

What is the status of new virtual disk? 

A: The status of the new disk is showing Ready.

Can you also ensure you have latest iDRAC, PERC and BIOS FW installed on the server before trying reconfigure?

A: iDRAC 7 Express. Version 2.65.65.65 (Build 15) Updated on Sept 8, 2020. I believe only version 7 is compatible with the Dell R620. This is a refurbished server I purchased from Amazon. 

BIOS version: 2.9

When I was installing ESXI 6.5 on this server I made sure to run the firmware upgrade in the BIOS itself so I believe whatever version I have should be the current version for all but I may be wrong. I have PERC H710 Mini on this server. 

I assume you are trying reconfigure from iDRAC? Is reconfigure option available from PERC Configuration Utility or OMSA?

Can anyone help?

Likely problem: You have multiple RAID 5's across the same disks. 

Called 'slicing', you cannot expand an array when multiple arrays exist on the same set of disks. This will be presented to the operating system as separate "disks".

Reconfigure also is not allowed if the disk type doesn't match (you can't mix SAS and SATA, HDD and SSD, etc.) OR if the disk is even slightly smaller (146.14GB vs 146.88GB) OR if you have a generic retail drive (instead of certified) that just plain doesn't talk to the controller correctly.

"I am using a Mac and cannot use OMSA I believe."

Use the right tool for the job. If you cannot manage your infrastructure from a Mac, why do you have one? Use a Windows box (if that is truly the reason why you can't.)

"When I was installing ESXI 6.5 on this server I made sure to run the firmware upgrade in the BIOS itself so I believe whatever version I have should be the current version for all"

Firmware updates are released all the time, so it's not a once and done thing - servers require regular maintenance and firmware updates are part of that.

I'm new to this and still learning. 

The disk type is SAS for all 4 drives (See below screenshot from iDRAC web UI). They might be of different brand though. 

Can they still not be reconfigured?

Regarding configuring it through OMSA, I am working on installing a windows server VM and see if I can install the OMSA there to reconfigure. The question is: Is this still doable using OMSA with the below fact?

"Called 'slicing', you cannot expand an array when multiple arrays exist on the same set of disks. This will be presented to the operating system as separate "disks"."

The reason you use certified drives is so that the make / model doesn't matter - all drives have the instruction set needed to communicate with the controller, regardless of make / model. Anything non-certified may or may not work. I think it is unlikely in this scenario, but this can NEVER be dismissed as a potential problem.

OMSA has to be installed on the physical server - it won't work in a VM. VMWare has a VIB with OMSA functionality ... if that is what is installed on the server.

You cannot expand a RAID array when multiple arrays are configured across the same set of disks, regardless of whether you use the controller's BIOS configuration utility or OMSA. How many "disks" does the OS see?

In any case, the above is only educational at this point. Your problem is the Foreign configuration on the new disk. You must CLEAR that Foreign configuration before you can do anything with that disk. Once you clear it, it will show as Ready, then, assuming you only have a single array across the disks, you can reconfigure the array to add a disk.

Can you share virtual disk page picture to verify whether you have multiple sliced virtual disks. As @theflash1932 mentioned new disk have foreign configuration. Until you clear it this disk can not be used as part of virtual disk. You can refer below video for clearing foreign configuration. You can also do the same from iDRAC as well.

https://www.youtube.com/watch?v=sDYeW5pp9eY 

@DELL-Shine K , please see the screenshot for virtual disk page on iDRAC:

 Can you please share the instruction to clear the foreign configuration using iDRAC? I couldn't find it.

You can use below link for details on clearing foreign configuration.

https://www.dell.com/support/manuals/en-us/idrac7-8-lifecycle-controller-v2.50.50.50/idrac_2.50.50.50_ug_pub/clearing-foreign-configuration?guid=guid-f31c2980-b845-4477-9b91-6fee3629d111&lang=en-us 

Note : It is recommended to take back up of data before doing such maintenance activity.

Hello @DELL-Shine K . Thanks for the link and prompt response. When I go to the Overview > Storage > Controllers > Setup, I see the Foreign configuration. It says Virtual Disk255 with capacity 2513.25 GB.

Q1: I am sorry for my lack of knowledge here but why is it showing 2513.25 GB when the new physical disk is of 837.75 GB? Is it adding the capacity to  the existing capacity?

Q2: This new hard drive is empty. If I clear the foreign configuration of this Virtual Disk255 with capacity 2513.25 GB, that means it'll clear the configuration for the new added drive and it won't affect the data and configuration of the existing drives, correct?

Q3: Can you direct me to an instruction page for how to backup data. This server has only VMware ESXI 6.5 as native OS. Will I still be able to backup the current virtual disk? Or can I install a Windows Server on the top of ESXI as a VM and backup?

@DELL-Shine K can you help answering my questions, please?